package Parse::RecDescent::FAQ; use vars qw($VERSION); $VERSION = '6.0.i'; 1; __END__ =head1 NAME Parse::RecDescent::FAQ - the official, authorized FAQ for Parse::RecDescent. =head1 OVERVIEW-TYPE QUESTIONS =head2 Is Parse::RecDescent LL(1)? LL(N)? LR(1)? LR(N)? =over 4 =item Answer by Yves Orton: I have a data structure which is a hash of entries where an entry is a list/array of sets I have also a grammar that can parse the syntax of the text files that contain the data I want to fill this structure with. Until here everything is ok. Problem: I cannot figure out how to actually FILL the parsed data into the structure. I can only decide if a string is grammatically correct or not. =back Also see the "Left-recursion" section under L<"PARSER BEHAVIOR"> =head1 DEBUGGING =head2 Flags you can set Paraphrased from Yves Orton: To understand more about why your parser is not behaving as it should take advantage of the follow variables: $::RD_ERRORS # unless undefined, report fatal errors $::RD_WARN # unless undefined, also report non-fatal problems $::RD_HINT # if defined, also suggestion remedies $::RD_TRACE # if defined, also trace parsers' behaviour =head2 Making warning line numbers correspond to your grammar How do I match the line numbers with the actual contents of my script? =head3 Solution 1 (from Holger Helmuth) A possible solution is to put anything more complicated than a trivial statement into a subroutine outside of the parser. To call it you have to qualify it with the package name. Without this (maybe trivial?) method I probably would have given up on P:RD on my last project. Disadvantage is that the code is distributed into two locations and there should be a speed penality too, but it was definitely worth it. =head3 Solution 2 At present, you can't (but that's on the ToDo list). Setting C<$::RD_TRACE> can be useful though: Once you've run with C<$RD_TRACE>, do this: perl -w RD_TRACE Then go and examine the actual line numbers given for the error in the file C. That will show you the actual generated code that's the problem. That code will, in turn, give you a hint where the problem is in the grammar (e.g. find out which subroutine it's in, which will tell you the name of the offending rule). =head1 PARSER BEHAVIOR =head2 Insuring a top-level rule match I have a question regarding subrules within grammars used with Parse::RecDescent - The following code best illustrates the best test case that I can identify which highlights my problem. Note that with this test code, each of the lines within the __DATA__ section are tested against the grammar and the expected result, either pass or fail, is indicated by the 1 or 0 at the start of the line respectively. However, despite these expected results, the grammar does not reject the lines arg1, and arg1,arg2,, which ideally should be rejected due to the incomplete match of the subrule (comma element) - In these cases, a trace shows that the subrule terminal comma is matched and consumed, despite the entire subrule, consisting of comma and element, not being matched. I am sure this is a relatively straight-forward oversight within the grammar on my part, but I am at a loss as to how to correct this. Program fragment delivered error ``couldnt open file : No such file or directory at ./tt.pl line 18, line 14.'' =head3 Answer by Randal L. Schwartz It's matching a portion of the string, which is legal unless you also anchor the end of the pattern. I typically use C at the end of my top-level pattern. Also, your comma-separated string can be parsed simply with argument: element(s /,/) as shown in the P::RD examples on the manpage. =head2 Floating or Substring Matches Oftentimes, you may want to find if a grammar pattern exists anywhere in the input string, not necessarily right from the start. In this post: L titled "Re: floating / backtracking - style match", Ted Zlatnov provides an elegant solution to getting the job done. =head2 Backtracking Take the following regex: /(aa|a)a/ It will match this text: aaa Now take the following PRD grammar: use strict; use warnings; use Parse::RecDescent; my $grammar = 'startrule: ( "aa" | "a" ) "a"'; my $parser = Parse::RecDescent->new($grammar); my $text = 'aa'; print defined($parser->startrule($text)) ? "Good!\n" : "Bad!\n"; It *will* print "Bad", meaning that PRD did not match it. The reason is that the first branch of the alternation matched, then the next subrule failed. You may ask: is there any way to persuade a top-down parser like P::RD to accept the above text as valid? I know that, in this simple example, I could easily rewrite the grammar (either by saying ("a" |"aa" ) "a" or "aa" a" | "a" "a" but what I mean is: Is there any additional feature I have missed which would allow the grammar as is to parse the text successfully? To put the question another way, can I get P::RD to behave more like a regex engine? After all, even an NFA engine would backtrack to try all possible alternatives before failing :-) (Perhaps parsers just do not backtrack past individual subrules under any circumstances.) =head3 And the answer is... RecDescent parsers do not work that way. They don't backtrack on failure; they just fail. Of course, there's nothing to prevent a recursive descent parser from incorporating backtracking too, but RecDescent doesn't. So, if you need backtracking in part of your grammar, you need to use plain old regexes there. =head2 Skipping over things The variable $Parse::RecDescent::skip is what you need. It takes strings and C-quoted regular expressions. See the Parse::RecDescent docs for details. Also, see the C parts of the docs. Here is an example of removing C comments in program text (courtesy Damian Conway) by treating them as whitespace! program: statement(s) statement: # etc... However, thanks to Chas Owens, we have an update that works for him. Instead of just C as a regular expression modifier he had to add s as well: program: statement(s) =head3 Quoting within a skip expression: Here is my script: ------ Start Script ------ use strict; use warnings; $::RD_TRACE = 1; use Parse::RecDescent; my $grammar = q{ input: number(s) { $return = $item{ number } } | number: /\d+/ }; my $parser = new Parse::RecDescent($grammar); my $test_string = qq{1.2.3.5.8}; print join( "\n", @{ $parser -> input( $test_string ) } ); ------ End Script ------ This script works great. However, if I change the value of the skip directive so that it uses double quotes instead of single quotes: the grammar fails to parse the input. However, if I put square brackets around the escaped dot: the grammar starts working again... =head4 and here's why: This small test program may help you figure out what's going wrong: print "\.*", "\n"; print '\.*', "\n"; Backslash works differently inside single and double quotes. Try: The reason the third variant: works is because it becomes the pattern: /[.]/ which is a literal dot. =head2 Left-recursion =over 4 =item * On elimination of left-recursion Randal Schwartz states: I had a fun time eliminating a mutual left-recursion problem for my "Data-Undumper" program, which used P::RD as an essential component. See my discussion of such at http://www.stonehenge.com/merlyn/LinuxMag/col29.html =item * Also, regarding elimination see this Perlmonks node: http://www.perlmonks.org/index.pl?lastnode_id=6364&node_id=153155 =item * Regarding detection of left-recursion, Conway states: RecDescent does a complete graph traversal looking for n-ary left-recursion loops and fails to compile the grammar if it finds any left-recursive loop involving any number of rules. It has done this since its earliest versions. =back =head2 Commit in subrule which is optional in rule Ques: When a subrule in a rule that has a "zero or more" repetition specifier (ie. ? or s?) has a directive in its production followed by the conditional production, if that subrule's production becomes committed, does that error cause the rule containing the subrule to fail also? It should right, if we have committed? It does not seem to work. Here is what I mean: myrule: 'stuff' mysubrule(?) mysubrule: ID '[' ']' | If the 1st production of mysubrule has committed, then myrule should fail. It doesn't seem to. If this is not a bug, how do I get this behavior? =head3 Answer by Damian The optional nature of the reference to mysubrule(?) means that, when the subrule fails (whether committed or not) the failure doesn't matter, since myrule can match if it finds zero mysubrules, which it just did. The usual way to get the rule-of-the-subrule to fail upon subrule failure is by "anchoring" the end of the match. That might be: myrule: 'stuff' mysubrule(?) ...!ID mysubrule: ID '[' ']' | or myrule: 'stuff' mysubrule(?) /\s*\Z/ mysubrule: ID '[' ']' | or whatever addition confirms that there really wasn't anything else after 'stuff'. =head3 Now that you think you know the answer... That answer is partially wrong, as was pointed out by Marcel Grunaer. In this phrase: myrule: 'stuff' mysubrule(?) ...!ID it is necessary to return a { 1 }, as the rule fails otherwise, presumably because of the negative lookahead: myrule: 'stuff' mysubrule(?) ...!ID { 1 } =head4 Marcel went on to point out an optimization: another option would be the use of a rulevar: myrule : myrule : 'stuff' mysubrule(?) mysubrule: ID '[' ']' | { $failed++ } this way you don't have to specify a potentially complex negative lookahead, and the method works over several levels of subrules as well. =head1 IGNORABLE TOKENS =head2 Removing C comments Since there is no separate lexer in recdescent. And it is top down. Is there anyway to deal w/ removing C comments that could be anywhere. =over 4 =item * Answer by Conway Sure. Treat them as whitespace! Do something like this: program: statement(s) statement: # etc... =back =head1 NEWLINE PROCESSING =head2 Parsing Windows Init (.ini) Files I'm trying ot use Parse::RecDescent to parse a configuration file which looks like this: [SECTION] parameter 1=value 1 parameter2=value 2 [NEXTSECTION] other parameter=other value =over 4 =item * Answer by Damian q{ config: section(s) section: section_label parameter(s) | section_label: /\[[A-Z]+\]\s*/ parameter: key "=" value key: /[A-Za-z0-9 ]+/ value: /.*\n/ } =item * Another Answer Also take a look at the example in section 11.2 in "Data Munging with Perl" by Dave Cross. =item * And the newest answer by the tireless Randal L. Schwartz: L =item * And this question pops up again on perlmonks 2007-08-16: L =back =head2 As end of line I'm trying to parse a text line by line using Parse::RecDescent. Each line is terminated by a "\n". Although the task setting up a grammar for this case is straightforward the following program doesn't produce any results. use Parse::RecDescent; $grammar = q{ line: word(s) newline { print "Found a line\n"; } word: /\w+/ newline : /\n/ }; $parse = new Parse::RecDescent ($grammar); $data = qq(This is line one\nAnd this is line two\n); $parse->line($data); RecDescent doesn't recognize the newlines. Does anybody know what I'm getting wrong? =over 4 =item * Answer by Damian By default, P::RD skips all whitespace (*including* newlines) before tokens. Try this instead: line: word(s) newline { print "Found a line\n"; } word: /\w+/ newline: /\n/ =back =head3 Example In a line-delimited grammar I want to use Parse::RecDescent to parse a certain assembly language. In assembly, data is "line oriented", that is newline is the statement separator. Simplified demonstration: command1 arg1 \n command2 arg2 \n Defines two statements. More than one statement on a line is illegal, a statement broken to two lines is also illegal. Now, P::RD has an issue with newlines. It swallows them by default. There is a workaround using the "skip" directive, but it became tedious and my grammar code is full of directives. Is there an easier way ? What I want, for starters, is a trivial "command arg" pairs for statements: line -> command arg "\n" =over 4 =item * Answer by Damian: tells RecDescent what to skip before trying each terminal. Normally that's any optional whitespace matched by the pattern C. But, if newlines (which would normally be skipped by the above pattern) are significant in your syntax, then you need to change the skipping pattern so that RecDescent doesn't skip newlines. And then match the newlines explicitly. For example: program : # Ignore non-newline whitespace statement(s) statement : command arg(s) "\n" command: /\w+/ args : /\d+/ | alphanum alphanum : /[A-Za-z0-9/ =back =cut =head1 COLUMN-ORIENTED PROCESSING =head2 Whitespace, text, column N, period, number (some reference to lookahead) Ok, now the line I'm trying to deal with is: "some amount of whitespace, then some text, then starting at column 48 a number, followed by a period, followed by another number". I want to capture the text (the "title"), and the two numbers (major and minor versions) =over 4 =item * Answer by Damian Conway You really do want to use a regex here (to get the lookahead/backtracking that RecDescent doesn't do). line: m/^(\s* # leading whitespace (.*?) # the title (?:\s+(?=\d+\.\d+\s*$)) # the space preceeding the numbers ) (\d+) # the major version number \. (\d+) # the minor version number /x { @{$return}{title major minor)} = ($2,$3,$4) } =back =head2 Another example I'm parsing some lines where the "column formatting" is fixed, i.e. a particular line might be formally described as "a single word followed by some amount of whitespace followed by another word whose first character begins at column 22". =over 4 =item * A simple answer that is wrong: Hmm, I guess I could make this simpler and do this: line: word word word: /\S+/ right? Wrong. And the reason why is that The won't skip any whitespace after the first word. You instead would want: line: word '' word =item * Restating it in the positive can be a GOTCHA: I'd state that in the positive instead: line: word '' { $thiscolumn == 22 } word This seems nice and more to the point, but unfortunately a failing conditional yields a false value but not necessarily an undef value. So in this case, you might get back a C<0> from evaluating this conditional, but unfortunately, that does not lead to failure. On the other hand, is exactly the same as the action { undef } and is guaranteed to make a production fail immediately. So if you would like to state the test in the positive, then do this: line: word '' { $thiscolumn == 22 || undef } word =back =head1 Parsing Quotes within .... Quotes! A recent node on Perlmonks L had a question on how to parse: A statement "with a quoted ';'" ; The best answer is the C section in the offical docs for L. An earlir node on Perlmonks: L covers the use of C in detail. =head1 MODULAR / GENERATIVE / CREATIVE / HAIRY PARSING Be sure to see the example see the L<"RESOURCES"> section at the end for even more nosebleed parsing examples. =head2 Subrule matches N times, where N is parsed on the fly Per this link: L we have a case where the future-matching is done as a function of the current matching. =head2 Rule-building by interpolating an array of choices Let's say you had to parse phrases like "6 red balls". You see this as C<"$quantity $color $item">. Now, the colors which are acceptable for parsing vary from program to program and so the rule which matches the colors needs to be variable. We see a fruitful discussion and solution of this problem here: L =head2 Macro processing In a RecDescent grammar, I'd like to add support in my language for "macros". These macros would be very much like #define macros in C and would support arguments that are instantiated into the expansion. In C, this looks something like: #define add(a,b) (a+b) In my language, I'd like something like: myMacro(x,y): any_expression where "any_expression" is any old (deep) expression defined elsewhere in the grammar, and C<(x,y)> would be replaced/instantiated in that expression. This may seem straighforward, but, well, it ain't to me. (I am, after all, mush4brains.) In my P::RD grammar, the "macro" rule without arguments would look like: macro: identifier ':' expr where, again, expr is defined fully in the grammar. Adding arguments to this... I need to instantiate my C<(x,y)> into the "expr" string before the subrules interpret it. Now... I thought of using standard Perl substitution in a pre-scan, but this seems less-than-ideal, since it requires some knowledge of the grammar in the Perl REs. I thought of using two separate grammars, the first of which is a simplified version of the full second grammar, but this also seems a bit redundant. =over 4 =item * And the answer is... http://perlmonks.org/index.pl?node_id=176399 =back =head2 Cloning parsers but giving each parser it own package It seems that the namespace for pre-compiled parsers (ie. compiled into a perl module) have hard-coded namespaces (IE. namespace00001). I was trying to clone one of these parsers by calling its 'new' method, but each parser is sharing the same namespace and thus any global variables I have in that namespace within a startcode block of my grammar before the first rule get overwritten by the other corresponding parser. =over 4 =item * Answer by Yves Orton Heres the required patch # Line 1692 of Parse::RecDescent reads my $nextnamespace = "namespace000001"; # Add This Method sub set_namespace { local $_=$_[1]; croak "$_ is not a legal namespace." if /[^\w]/; $_.="00" unless /\d+$/; # Ensure our namespace has a number at the end $nextnamespace=$_; } # And then just call Parse::RecDescent->set_namespace("MyNameSpace"); =back =head2 Parsing sentences to generate sentences In this column, Randal shows how to read text to generate more text. He parses sentences to make Parse::RecDescent parse trees which he then re-walks with random weightings to create new sentences. http://www.stonehenge.com/merlyn/LinuxMag/col04.html =head2 Calling a parser within a grammar I have a script that uses Parse::RecDescent, in which I want to define 2 parsers. The grammar for the second parser has to call the first parser. Can I do this? =over 4 =item * yes, here's an example #!/usr/local/bin/perl -w use strict; use Parse::RecDescent; $::RD_ERRORS = 1; $::RD_WARN = 1; $::RD_HINT = 1; our $text_to_parse = ""; my $grammar1 = q{ [...] } our $inner_parser = new Parse::RecDescent($grammar1); my $grammar2 = q{ [...] rule: TEXT { $text_to_parse = $item{TEXT}; if (defined $text_to_parse) { print "executing inner parse...\n"; } my $p_text = $inner_parser->startrule($text_to_parse); } [...] } =back =head2 Incremental generation of data structure representing parse I have a data structure which is a hash of entries where an entry is a list/array of sets I have also a grammar that can parse the syntax of the text files that contain the data I want to fill this structure with. Until here everything is ok. Problem: I cannot figure out how to actually FILL the parsed data into the structure. I can only decide if a string is grammatically correct or not. =over 4 =item * Answer by Marcel Grunaer Try this grammar, which you have to feed the input as one big string. It uses a global variable, $::res into which the results are assembled. At the end the variable is also returned for convenience. It basically parses a phrase and a list of meanings. Instead of reconstructing what it just parsed at each step, it checks the remaining text at various stages (using an idea taken from Parse::RecDescent::Consumer) to see what the 'phrase' or 'meaning' subrules just matched. The 'meanings' subrule then (implicitly) returns a reference to an array of 'meaning' strings. That arrayref is stored at the proper slot in the result hash. (Hope that explanation makes sense. I'm sure Damian can come up with a grammar that's way more elegant and efficient...) { sub consumer { my $text = shift; my $closure = sub { substr $text, 0, length($text) - length($_[0]) } } } start : entry(s) { $::res } entry : comment | def | def : def : # The // skips initial whitespace so it won't end up in $p_text def : // { $p_cons = consumer($text) } phrase { $p_text = $p_cons->($text) } '=' meanings ';' { $::res->{$p_text} = $item{meanings} } comment : /#.*(?=\n)/m phrase : ident(s) ident : /[\w&\.'-]+/ meanings : meaning(s /:/) meaning : meaning : // { $m_cons = consumer($text) } element(s /,?/) { $m_cons->($text) } element : alternation(s /\|/) alternation : expr(s /[+>]/) expr : /!?/ term term : ident '(' meaning ')' | ident =back =head2 XOR as opposed IOR alternation matching I'm using alternations in some productions but in contrast to the definition of the |-operator, I'm looking for a behaviour which is XOR (^) not OR (|). So far I used the directive to simulate such a behaviour. Is there any easy solution to this? =over 4 =item * Answer by Randal Schwartz Use a set. use Parse::RecDescent; use Data::Dumper; $|++; my $parser = Parse::RecDescent->new(q{ line: word(s) /\z/ { my @words = @{$item[1]}; my %count; (grep ++$count{$_} > 1, @words) ? undef : \@words; } word: "one" | "two" | "three" }) or die; for ("one two", "one one", "two three one", "three one two one") { print "$_ =>\n"; print Dumper($parser->line($_)); } # which generates: one two => $VAR1 = [ 'one', 'two' ]; one one => $VAR1 = undef; two three one => $VAR1 = [ 'two', 'three', 'one' ]; three one two one => $VAR1 = undef; =item * Embellishment by Damian Conway Furthermore, if uniqueness was something you needed to enforce more widely in your grammar, you could factor it out into a parametric rule: use Parse::RecDescent; use Data::Dumper; $|++; my $parser = Parse::RecDescent->new(q{ line: unique['word'] /\z/ { $return = $item[1] } unique: (s) { my %seen; foreach (@{$item[1]}) { undef $item[1] and last if $seen{$_}++ } $return = $item[1]; } word: "one" | "two" | "three" }) or die; for ("one two", "one one", "two three one", "three one two one") { print "$_ =>\n"; print Dumper($parser->line($_)); } =back =head1 CLEANING UP YOUR GRAMMARS In honor of the original (and greatest) Perl book on cleaning up your Perl code, this section is written in the style of Joseph Hall's "Effective Perl Programming" =head2 Use repetition modifiers with a separator pattern to match CSV-like data The intuitive way to match CSV data is this: CSVLine: NonFinalToken(s?) QuotedText NonFinalToken: QuotedText Comma { $return = $item[1] } or, in other (merlyn's) words, "many comma terminated items followed by one standalone item". Instead, take the approach shown by merlyn: CSVLine: QuotedText(s Comma) { use Data::Dumper; Dumper($item[1]) } Then just define C, C, and you're done! =head1 OPTIMIZING YOUR GRAMMARS =head2 Dan Sugalski's Practice and Theory of Optimizing PRD Grammars This link is excellent. So excellent in fact, that I took the time to mirror it to my site to avoid it ever being lost: L This link provides an excellent discussion of hits and misses in optimizing a grammar whose initial parse time started out at around minutes. After whittling it down to 5 minutes, Dan is still not happy and continues with even more aggressive optimizations! Well worth the read. =head2 Eliminate backtracking when possible Let's take a look at two computationally equivalent grammars: expression : unary_expr PLUS_OP expression | unary_expr versus expression : unary_expr plus_expression plus_expression : PLUS_OP expression | # nothing The second one is more efficient because it does not have to do backtracking. The first one is more readable and more maintainable though. It is more readable because it doesnt have an empty rule. It is more maintainable because as you add more expression types (minus_expression, mult_expression...) you don't have to add an empty rule to each of them. The top level description scales without change. But, if speed is what you want then the second one is the way to go. =head2 Precompiling Grammars for Speed of Execution Take a look at Parse::RecDescent's precompilation option under the section titled "Precompiling parsers". =head2 Parse::RecDescent is slow on Really Big Files. How can I speed it up? =over 4 =item * Reduce the "depth" of the grammar. Use fewer levels of nested subrules. =item * Where possible, use regex terminals instead of subrules. For example, instead of: string: '"' char(s?) '"' char: /[^"\\]/ | '\\"' | '\\\\' write: string: /"([^"\\]|\\["\\])*"/ =item * Where possible, use string terminals instead of regexes. =item * Use repetitions or / instead of recursion. For example, instead of: list: '(' elems ')' elems: elem ',' elems | elem write: list: '(' ')' or: list: '(' elem(s /,/) ')' =item * Factor out common prefixes in a set of alternate productions. For example, instead of: id: name rank serial_num | name rank hair_colour | name rank shoe_size write: id: name rank (serial_num | haircolour | shoe_size) =item * Pre-parse the input somehow to break it into smaller sections. Parse each section separately. =item * Precompile they grammar. This won't speed up the parsing, but it will speed up the parser construction for big grammars (which Really Big Files often require). =item * Consider whether you would be better porting your grammar to Parse::Yapp instead. =back =head1 CAPTURING MATCHES =head2 Hey! I'm getting back ARRAY(0x355300) instead of what I set $return to! Here's a prime example of when this mistake is made: QuotedText: DoubleQuote TextChar(s?) DoubleQuote { my $chars = scalar(@item) - 1; $return = join ('', @item[2..$chars]) } This rule is incorrectly written. The author thinks that C<@item> will have one C from position 2 until all Cs are matched. However, the true structure of C<@item> is: =over 4 =item position one: the string matched by rule DoubleQuote =item position two: array reference representing parse tree for TextChar(s?) =item position three: the string matched by rule DoubleQuote =back Note that position two is an array reference. So the rule must be rewritten in this way. QuotedText: DoubleQuote TextChar(s?) DoubleQuote { $return = join ( '', @{$item[2]} ) } =head2 Getting text from subrule matches I can't seem to get the text from my subrule matches... =over 4 =item * Answer by Damian Conway Your problem is in this rule: tuple : (number dot)(2) is the same as: tuple : anon_subrule(2) anon_subrule : number dot Like all subrules, this anonymous subrule returns only its last item (namely, the dot). If you want just the number back, write this: tuple : (number dot {$item[1]})(2) If you want both number and dot back (in a nested array), write this: tuple : (number dot {\@item})(2) =back =head2 Capturing whitespace between tokens I need to capture the whitespace between tokens using Parse::RecDescent. I've tried modifying the $skip expression to // or /\b/ (so I can tokenize whitespace), but that doesn't seem to have the desired effect. Just having a variable where all skipped whitespace is stored would be sufficient. Does anybody know how to trick Parse::RecDescent into doing this? =over 4 =item * Answer by Damian Conway To turn off whitespace skipping so I can handle it manually, I always use: See: demo_decomment.pl demo_embedding.pl demo_textgen.pl for examples. =back =head2 My grammar is not returning any data! What's wrong?! =over 4 =item * Answer by Brent Dax: This is a clue; either something is wrong with your actions or the grammar isn't parsing the data correctly. Try adding | clauses to the end of each top-level rule. This will tell you if there's a parsing error, and possibly what the error is. If this doesn't show anything, look hard at the actions. You may want to explicitly set the $return variable in the actions. =back =head1 THINGS NOT TO DO =head2 Don't think that rule: statement and rule: statement(1) are the same Even though in pure Perl, the repetition modifier returns the same data structure without or without an argument of one: use Data::Dumper; my @a = (x); my @b = (x) x 1; my $x = 'x'; my $y = 'x' x 1; In this first case below, C returns a scalar upon matching, while C returns an arrayref with 1 element upon matching: rule_one: statement rule_two: statement(1) However counter-intuitive this may at first sound, Damian provides us with some insight: I don't think C is the right analogy for RecDescent repetition operators. The behaviour of C<*> and C<+> in regexes is a closer model. I guess it depends on your poitn of view. I would have said that the absolute consistency with which *every* repetition (regardless of its number) returns an array ref is better than the alternative: christmas: gold_rings(5) # returns array ref calling_birds(4) # returns array ref French_hens(3) # returns array ref turtle_doves(2) # returns array ref partridge_in_a_pear_tree(1) # returns scalar value Especially if you have to change the count at some later point, which would mess up any code relying on the type of value returned. =head2 Do not follow with to skip errors C is used to allow a rule which would normally fail to "pass" so that parsing can continue. If you add the reject, then it unconditionally fails. =head2 Do not assume that %item contains an array ref of all text matched for a particular subrule For example: range: '(' number '..' number )' { $return = $item{number} } will return only the value corresponding to the last match of the C subrule. To get each value for the number subrule, you have a couple of choices, both documented in the Parse::RecDescent manpage under C<@item and %item>. =head2 use not If you say: somerule: you get a lexical $x within the rule (only). If you say: somerule: you get a localized $x within the rule (and any subrules it calls). =head2 Don't use $::RD_AUTOACTION to print while you are parsing You can't print out your result while you are parsing it, because you can't "unprint" a backtrack. Instead, have the final top-level rule do all the diagnostic printing, or alternatively use P::RD's tracing functionality to observe parsing in action. =head1 ERROR HANDLING =head2 Propagating a failure up after a on a subrule See L =head2 In a non-shell (e.g. CGI) environment I need to parse a file with a parser that I cooked up from Parse::Recdescent. My problem, it must work in a cgi environment and the script must be able to handle errors. I tried eval, piping, forking, Tie::STDERR, but the errors msgs from Parse::Recdescent seem unstoppable. I can catch them only by redirecting the script's STDERR from the shell. But howdo I catch them from within ?? =over 4 =item * Like this: use Parse::RecDescent; open (Parse::RecDescent::ERROR, ">errfile") or die "Can't redirect errors to file 'errfile'"; # your program here =back =head2 Accessing error data What I want to do is export a condition to the calling program. Currently _error writes the error message to (what essentially is STDOUT) which means a parsing error prints a nice message, but it is up to the reader to DO anything about it. I have a Tk viewer that will parse the file. When the parse fails, I would like to capture the line number and error message returned and position the viewer (a text widget) to that line, highlighted. And no STDOUT message. Something kind of like the eval function where the return value is the result but $@ is set as a "side effect". Am I missing something about the built in capability? Is the solution as simple as overloading the Parse::RecDescent::_error subroutine with my own copy which might look like this: %Parse::RecDescent::ERROR=(); ... sub Parse::RecDescent::_error($;$) { $ERRORS++; return 0 if ! _verbosity("ERRORS"); %Parse::RecDescent::ERROR=('line'=>$_[1],'msg'=>$_[0]); return 1; } It seems like it should work and I tried it and it did. BUT this is an extremely complex bit of code and I'm concerned about unforseen consequences. Of course, I could make ERROR a my variable and provide a method to access it so it is not a "global", and this would be OO code of a higher purity (or something), but that is not really the point. =over 4 =item * Answer by Damian Conway You can get access to the error data by referring to the attribute C<$thisparser->{errors}> within a rule. C<$thisparser->{errors}> is a reference to an array of arrays. Each of the inner arrays has two elements: the error message, and the line number. So, for example, if your top-level rule is: start: subrule1 subrule2 then you could intercept any errors and print them to a log file, like so: start: subrule1 subrule2 | { foreach (@{$thisparser->{errors}}) { print LOGFILE "Line $_->[1]:$_->[0]\n"; } $thisparser->{errors} = undef; } Note that the line: $thisparser->{errors} = undef; is doing double duty. By resetting C<$thisparser->{errors}>, it's preventing those annoying error messages from being automagically printed. And by having the value C as the last value of the action, it's causing the action to fail, which means the second production fails, which means the top rule fails, which means errors still cause the parse to fail and return C. =back =head2 Simple Error Handling I'm trying to write a parser for orders for Atlantis (PBEM game). Syntax is pretty simple: one line per command, each command starts with name, followed by list of parameters. Basically it's something like this (grammar for parsing one line): Statement:Comment | Command Comment(?) Comment:/;.*/ Command:'#atlantis' FactionID String Command:'attack' Number(s) .... However I have problems to make it work as I want: 1) In case of failed parsing (syntax error, not allowey keyword, ...) I want to store error messages in variable (not to be just printed), so I can process them later. I don't think Parse::RecDescent has a hook for that (Damian, something for the todo list?), but you can always install a $SIG {__WARN__} handler and process the generated warnings. 2) In case if user types "attack bastards" I want to give him error message that "list of numbers expected" instead of just saying the "cannot parse this line". The only thing that I came up with now was defining every command like this: Command:Attack Attack:'attack' AttackParams AttackParams:Number(s) | ... Any better solutions? =over 4 =item * You can just do: Command: '#atlantis' FactionID String | 'attack' Number(s) | and when you try to parse "attack bastards", you will get: ERROR (line 1): Invalid Command: Was expecting Number but found "bastards" instead You might want to use , which will only print the error when it saw '#atlantis' or 'attack' (because then you are committed). =back =head2 Collecting all error messages for processing Is there a way to to collect the parser-generated errors and use them later on in my script? =over 4 =item * Answer by Damian There's no "official" way (yet). A future release will allow you to create pluggable OO error handlers. At the moment the best you can do is: startrule: try_this | try_that | try_the_other | { ::do_something_with( $thisparser->{errors} ) } $thisparser->{errors} will contain a reference to an array-of-arrays, where each inner array contains the error message and the line number it occurred at. =back =head1 OTHER Parse::RecDescent QUESTIONS =head2 How to get the old perl.com article to work? Jethro of Perlmonks answers: L The problem is that C finishes parsing. Parse::RecDescent works with a variable C<$return> to give back results from subrules to parent rules. If you change the three occurences of C inside the grammar (but not inside the sub 'expression') to C<< $return= >> the parser works. =head2 Matching line continuation characters I need to parse a grammar that includes line continuation characters. For example: // COMMAND ARG1-VALUE,ARG2-VALUE, + ARG3-VALUE,ARG4-VALUE, + EVEN-MORE-ARGS // ANOTHERCOMMAND * and a comment * or two How do I formulate a rule (or rules) to treat the first command as if all 5 arguments were specified on a single line? I need to skip over the /\s*+\n\s*/ sequence. It seems like skip or resync should do this for me, but if so, I haven't discovered the correct technique, yet. =over 4 =item * Answer by Damian Conway use Parse::RecDescent; my @lines = << 'EOINST'; // COMMAND ARG1-VALUE,ARG2-VALUE, + ARG3-VALUE,ARG4-VALUE, + EVEN-MORE-ARGS // ANOTHERCOMMAND * and a comment * or two EOINST my $parse = Parse::RecDescent->new(join '', ) or die "Bad Grammar!"; use Data::Dumper 'Dumper'; print Dumper [ $parse->Instructions("@lines") or die "NOT parsable!!\n" ]; __DATA__ Instructions: command(s) command: multiline_command | singleline_command | comment singleline_command: '//' /.*/ { {command => $item[-1]} } multiline_command: '//' /(.*?[+][ \t]*\n)+.*/ { $item[-1] =~ s/[+][ \t]*\n//g; {command => $item[-1]} } comment: '*' /.*/ { {comment => $item[-1]} } =back =head2 How can I match parenthetical expressions to arbitrary depth? Example: a, (b ,c, (e,f , [h, i], j) ) =over 4 =item * Answer by FAQ author Maybe Text::Balanced is enough for your needs. See it on search.CPAN.org under author id DCONWAY. =item * Answer by lhoward of perlmonks.org: Parse::RecDescent implements a full-featured recursive-descent parser. A real parser (as opposed to parsing a string with a regular expression alone) is much more powerful and can be more apropriate for parsing highly structured/nested data like you have. It has been a while since I've written a grammer so it may look a bit rough. use Parse::RecDescent; my $teststr="blah1,blah2(blah3,blah4(blah5,blah6(blah7))),blah8"; my $grammar = q { content: /[^\)\(\,]+/ function: content '(' list ')' value: content item: function | value list: item ',' list | item startrule: list }; my $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n"; defined $parser->startrule($teststr) or print "Bad text!\n"; To which merlyn (Randal Schwartz) of perlmonks.org says: Simplifying the grammar, we get: use Parse::RecDescent; my $teststr="blah1,blah2(blah3,blah4(blah5,blah6(blah7))),blah8"; my $grammar = q { list: item: word '(' list ')' | word word: /\w+/ }; my $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n"; defined $parser->list($teststr) or print "Bad text!\n"; =back =head2 Switching out of first-match-wins mode I have a set of alternatives on which I want to avoid the default first-match-wins behavior of Parse::RecDescent. How do I do it? =over 4 =item * Answer by FAQ author Use a scored grammar. For example, this scoring directive opcode: /$match_text1/ opcode: /$match_text2/ opcode: /$match_text3/ would return the opcode with the longest length, as opposed to which one matched first. Just look for the section "Scored productions" in the .pod documentation. =back =head2 I'm having problems with the inter-token separator: my $parse = Parse::RecDescent->new(<<'EndGrammar'); rebol : block { dump_item('block', \@item) } | scalar { dump_item('scalar', \@item) } block : '[' block_stuff(s?) ']' block_stuff : scalar scalar : '%' file file : /w+/ EndGrammar My grammar matches a filename, ie: %reb.html just fine. However, it does not match a filename within a block, ie: [ %reb.html ] and I know exactly why after tracing the grammar. It is trying the '%' file production with the input text " %reb.html" note the space in the input text. The reason this distresses me is that I have not changed the universal token separator from /\s*/ Yet it did not gobble up the white space between the '[' terminal and the '%' file production =over 4 =item * Answer by Randal Schwartz That's the expected behavior. The outer prefix is in effect until changed, but you changed it early in the rule, so the previous "whitespace skip" is effectively gone by the time you hunt for '%'. To get what you want, you want: '%' file in your rule. back =back =head2 Matching blank lines How do I match an arbitrary number of blank lines in Parse::RecDescent? =over 4 =item * Answer by Damian Conway Unless you use the /m suffix, the trailing $ means "end of string", not "end of line". You want: blank_line: /^\s+?$/m or blank_line: /^\s+?\n/ =back =head2 More on blank lines I have a rule which MUST be failing, but it isn't. Why? blank_line: { $text =~ /silly-regex/ } parses with no error. =over 4 =item * Answer by Damian Conway The pattern match still fails, but returns the empty string (""). Since that's not undef, the rule matches (even though it doesn't do what you want). =back =head2 How can I get at the text remaining to be parsed? See the documentation for the C<$text> variable. =head2 You don't escape Perl symbols in your grammars. Why did I have to? my $grammar = < EOGRAMMAR Why does '@' have to be escaped? And whatever reason that may be, why doesnt it apply to '\n'? =over 4 =item * Answer by Damian Conway Because you're using an interpolating here document. You almost certainly want this instead: my $grammar = <<'EOGRAMMAR'; # The quotes are critical! export_line: stock_symbol COMMA # 1 stock_name COMMA2 # 2 stock_code COMMA3 # 3 trade_side COMMA4 # 4 trade_volume COMMA5 # 5 floating_point COMMA6 # 6 tc # 7 { print "got @item\n"; } | EOGRAMMAR =back =head2 Other modules appear to not work when used with P::RD Such-and-such a module works fine when I don't use Parse::RecDescent =over 4 =item * Answer by Damian Conway Did you alter the value of undef with your parser code? The problem has nothing to do with Parse::RecDescent. Rather, it was caused by your having set $/ to undef, which seems to have caused Mail::POP3 to over-read from its socket (that might be considered a bug in the Mail::POP3 module). As a rule-of-thumb, *never* alter $/ without local-izing it. In other words, change things like this: $/ = undef; to this: { local $/; } =back =head1 PROGRAMMING TOPICS GERMANE TO Parse::RecDescent USE =head2 Getting large files into memory An issue related to PRD is how to get huge files into memory for parsing. Check out this link: L for some approaches to this problems. =head2 Double vs Single-quoted strings I'm playing around with the directive and I've noticed something interesting that I can't explain to myself. Here is my script: Program fragment delivered error ``couldnt open file : No such file or directory at ./tt.pl line 18, line 332.'' This script works great. However, if I change the value of the skip directive so that it uses double quotes instead of single quotes: the grammar fails to parse the input. However, if I put square brackets around the escaped dot: the grammar starts working again: How does this work this way? =over 4 =item * Damian says: This small test program may help you figure out what's going wrong: print "\.*", "\n"; print '\.*', "\n"; Backslash works differently inside single and double quotes. Try: The reason the third variant: works is because it becomes the pattern: /[.]/ which is a literal dot. =back =head2 Tracking text parsed between phases of the parse I wanted to know, after matching a rule, what text the rule matched. So I used two variables to remember what the remaining text and offset were before and after the rule and just determined the difference. report : report : report : { $rule_text = $text; $rule_offset = $thisoffset; } ...some subrules... { my $str = substr($rule_text, 0, $thisoffset - $rule_offset); # remove all sorts of whitespace $str =~ s/^\s*//s; $str =~ s/\s*$//s; $str =~ s/\s+/ /gs; # Now $str contains the text matched by this rule } This is the kind of thing I thought would have been possible a lot easier. Did I miss something? If not, is there a way to make this available in every parser, e.g. by providing a new directive or something like that? =over 4 =item * The answer is on CPAN Parse::RecDescent::Consumer, on CPAN, prints out the text consumed between stages of a parse... even if that part may fail later. The implementation is straightforward, it creates closures containing C<$text> and evaluates them later to get the text consumed. =back =head2 Unconditionally listifying scalars Quite often when using Parse::RecDescent, I want to treat the return value of a production the same regardless of whether P::RD returns a string or a list of string. =over 4 =item * Use Scalar::Listify from CPAN. =back =head2 Shallow versus Deep Copying =head3 An article by Randal Schwartz L =head3 A tutorial written by Philip Newton Start off with an array of (references to) arrays: @array = ( [1,2,3], ['a', 'u', 'B', 'Q', 'M'], ['%'] ); Now a shallow copy looks like this: @shallow = ( $array[0], $array[1], $array[2] ); This copies the references over from @array to @shallow. Now @shallow is ( [1,2,3], ['a', 'u', 'B', 'Q', 'M'], ['%'] ) -- the same as @array. But there's only one 2 and one 'Q', since there are two references pointing to the same place. Here's what it looks like in the debugger: DB<5> x \@array 0 ARRAY(0x10e5560) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'B' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' DB<6> x \@shallow 0 ARRAY(0xcaef60) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'B' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' You can see that @array lives somewhere around 0x10e5560, whereas @shallow lives around 0xcaef60, but the three references point to arrays in the same place. If I now change $array[1][2] to 'C', watch what happens: DB<7> $array[1][2] = 'C' DB<8> x \@array 0 ARRAY(0x10e5560) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' DB<9> x \@shallow 0 ARRAY(0xcaef60) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' $shallow[1][2] is now also 'C'! This is because it just followed the pointer to the array at 0x10e5638 and found the modified data there. Now see what happens when I do a copy that's one level deeper -- not just copying the references but the data behind the references: @deep = ( [ @{$array[0]} ], [ @{$array[1]} ], [ @{$array[2]} ] ); This uses the knowledge that @array[0..2] are all references to arrays, and it only goes one level deeper. A more general algorithm (such as Storable's dclone, mentioned in `perldoc -q copy`) would do a walk and copy differently depending on the type of reference it encounters at each stage. Now watch: DB<12> x \@array 0 ARRAY(0x10e5560) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' DB<13> x \@deep 0 ARRAY(0x10ef89c) 0 ARRAY(0x10eb298) 0 1 1 2 2 3 1 ARRAY(0x10eb2c4) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10ef07c) 0 '%' The references point to different places. Now if you change @array, @deep doesn't change: DB<14> push @{$array[2]}, '$' DB<15> x \@array 0 ARRAY(0x10e5560) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' 1 '$' DB<16> x \@shallow 0 ARRAY(0xcaef60) 0 ARRAY(0x10e5464) 0 1 1 2 2 3 1 ARRAY(0x10e5638) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10e568c) 0 '%' 1 '$' DB<17> x \@deep 0 ARRAY(0x10ef89c) 0 ARRAY(0x10eb298) 0 1 1 2 2 3 1 ARRAY(0x10eb2c4) 0 'a' 1 'u' 2 'C' 3 'Q' 4 'M' 2 ARRAY(0x10ef07c) 0 '%' @deep didn't change, since it's got its own value of the anonymous array containing '%', but @shallow did. Hope this helps a bit. Cheers, Philip -- Philip Newton If you're not part of the solution, you're part of the precipitate =head2 Apparent, but not really deep copying: my (@list) = @{[@{$_[0]}]}; I was meandering through demo_calc.pl in the Parse::RecDescent demo directory and came across this sub evalop { my (@list) = @{[@{$_[0]}]}; my $val = shift(@list)->(); ... } I took the line that confused me step-by-step and don't get the purpose of this. Working from inner to outer: @{$_[0]} # easy --- deference an array reference [ @{$_[0]} ] # ok --- turn it back into an array ref.. why? @{ [ @{$_[0]} ] } # umm -- uh.... well, the @sign implies # we have an array, but how is it # different from the first array we # dereferenced? =over 4 =item * Matthew Wickerline says The line from demo_calc.pl is in fact not doing any deep copying. #!/usr/bin/perl -w my @original = ( [0], [1,2,3], [4,5,6], [7,8,9] ); my @copy = &some_kind_of_copy( \@original ); sub some_kind_of _copy { # here's that line from demo_calc.pl my (@list) = @{[@{$_[0]}]}; return @list; } $original[0][0] = 'zero'; @{ $original[1] }[0..2] = qw(one two three); @{ $original[2] }[0..2] = qw(four five six); @{ $original[3] }[0..2] = qw(seven eight nine); # now use the debugger to look at the addresses, # or use Data::Dumper to look at @copy, or just # compare one of the items... if ( $copy[1][2] eq 'three' ) { print "Shallow Copy\n"; } elsif ( $copy[1][2] == 3 ) { print "Deep Copy\n"; } else { print "This should never happen!!!\n" } If you wanted that line to do deep copying of a list of anon arrays, then the line should read my @list = map { [@$_] } @{$_[0]}; # turn $_[0] into a list (of arrayrefs) # turn each (arrayref) element of that list # into an anonymous array containing # a list found by derefrencing the arrarref Try plugging that line into above script instead of the line from the demo_calc.pl and you'll see different output. The line from demo_calc.pl is in fact doing extra useless work. My guess is that the extra @{[ ]} around there is one of two things: 1) a momentary lapse of attention resulting in a copy/paste error, or duplicate typing or 2) an artifact of earlier code wherein something extra was going on in there and has since been deleted. Even Damian can make a mistake, but it's not a mistake that affects output... it just makes for a tiny bit of wasted work (or maybe Perl is smart enough to optimze away the wasted work, I dunno). =item * Damian Conway says: I have no recollection of why I did this (see children, that's why you should *always* comment your code!). I *suspect* it's vestigal -- from a time when contents of the argument array reference were somehow modified in situ, but it was important that the original argument's contents not be changed. The ungainly C<@{[@{$_[0]}]}> syntax is a way of (shallow) copying the array referenced in $_[0] without declaring a new variable. So another possible explanation is that evalop may originally have been a one-liner, in which case I might have used this "inlined copy" to keep the subroutine's body to a single expression. However... Even Damian can make a mistake is by far the likeliest explanation. =back =head2 Regular Expressions =head3 Shortest match instead of longest match What is the Perl idiom for getting the leftmost shortest match? For instance, so: $a = "banana"; $a =~ /b.*n/; # but different print $&; yields C. How can I get it to yield C? =over 4 =item * Just use the non-greedy quantifier modifier, C : $a =~ /b.*?n/; =back =head3 Tutorial on positive and negative lookahead and lookbehind regexps: http://lists.isb.sdnpk.org/pipermail/comp-list/2002-August/001156.html =head3 Japhy's Book on Regular Expressions Ok, so he hasn't named it yet. It's available online for free! http://japhy.perlmonk.org/book =head3 "Mastering Regular Expressions" by Jeffrey Freidl You still need to know when to use C or C or C =head1 Perlmonks Nodes Basically the number of posts on Perlmonks about Parse::RecDescent are growing faster than I have time to study them closely and categorize them. But it is important that you have rapid access to them instead of slogging through Super Search or a google site search, so here you go: =over 4 =item * Pro teaches amateur L Perlmonk Ikegami gives a wonderful set of best practices for Recdescent usage. =back =head1 RESOURCES =head2 Web Links =head3 "Some Parse::RecDescent Tutorials" L =head3 Craig's Parse::Recdescent Area Contains an article (in English) published during the Zweiter Perl Workshop as well as some slides for a presentation: http://www.informatik.uni-trier.de/~smith/perl/recdescent/ =head3 Hugh Myer's tips on Parse::RecDescent http://www.perlmonks.org/index.pl?node_id=180778 17 tips you cannot do without... recently updated! =head3 Parse::Recdescent tutorial at www.perl.com http://www.perl.com/pub/a/2001/06/13/recdecent.html Yes, I see the mis-spelling, but the link works. And in the Perl community, we are all about pragmatism! =head2 Published articles and books =head3 Parse Data::Dumper output In the Linux Magazine article "Safe undumping" by Randal Schwartz uses Parse::RecDescent to parse Data::Dumper output. Not fast, but quite complete. =head3 "Parsing Interesting Things" SysAdminMag.COM, December 2001, Randal Schwartz =head3 "Data Munging with Perl" by Dave Cross Chapter 11 is focused on parsing with special emphasis on practical use of Parse::RecDescent. =head3 "Object-Oriented Perl" by Damian Conway This book will aid you in complexity management for large grammars. =head2 CPAN Modules of Wonderful Utility! =head3 Data::Match L is an excellent tool for parse tree foraging. A typical autoaction is $::RD_AUTOACTION = q { [@item] } ; which leads to deeply nested parse trees. I first inquired about various tools for spelunking in such trees: http://perlmonks.org/?node_id=646560 And am very happy so far with Data::Match. Here is a sample parse tree from my parse: $VAR1 = [ 'simple_house', [ 'pre_simple', [ 'geoloc', [ 'geoloc_', [ 'place', [ 'city', 'taipei' ] ] ] ] ], [ 'house', 'house' ], [] ]; Now, my goal is to get at the inner-most array in $VAR1->[1]: [ 'city', 'taipei' ] And with Data::Match, I can do so in a very definitional fashion: my $match = match ( # The parse tree $self->{parse_result}[1], # The Data::Match pattern match template FIND ( COLLECT ( 'x', [ EXPR(q{! ref}), EXPR(q{! ref}) ] ) ) ); The pattern is basically saying: "match an array ref consisting of 2 elements where each element is not an reference of any sort". Since the strings 'city' and 'taipei' both fulfill that criterion that is what matches. =head3 Data::MultiValuedHash Transparent manipulation of single or multiple-valued Perl hash values. =head3 Parse::RecDescent::Consumer L makes it easy to keep track of how much text has been consumed through successful matches. =cut =head3 Regexp::List (part of Regexp::Optimizer) If you have a lot of alternations, it will be more efficient to test a single pre-compiled, optimized regular expression than a bunch of strings. And L is designed to help you do just that. =head3 Regexp:Assemble L is a similar tool to Regexp::List, except that instead of just providing plain strings, you can give it a series of regular expressions, for it to assemble into one large regular expression. =head3 Text::Balanced and Regexp::Common::balanced Use this instead of writing hairy regular expressions to match certain common "balanced" forms of text, such as tags and parenthesized text. See http://perlmonks.org/index.pl?node_id=208285 for an example of where it would be better suited for the task than Parse::RecDescent. =head3 Scalar::Listify Sometimes in grammars, you are not sure if you will end up with a scalar match or an array of results. But you will often want to process them both the same. L facilitates this process. =head2 Practical Parser Examples =head3 DBFeeder / Data::Generate - database test data generation tool L is a a powerful data generation tool. Internally, it uses L to parse "inverted regular expressions" using Parse::RecDescent. =head4 Tangential new directions In a recent perlmonks thread: L the author of DBFeeder (Davide Conti) discusses SQL parsing with Parse::RecDescent (among other tools). This technology is not currently a part of DBFeeder and Data::Generate, but an avenue he is exploring. =head3 Article comparing Gene Parsing with PRD and other approaches "The Making of Entrez Gene parsers in Perl using Parse::RecDescent, Parse::Yapp, Perl-byacc and Regex" by Dr. Mingyi Liu is available from: L =head3 Module dependency expression parser This code by Marc Prewitt was a response to Mark Dominus' Perl 'Expert' Quiz of the Week #24 (Module dependency evaluation). This quiz of the week was a practical challenge to find code that could end up useable within Module::Build. The requirement was to allow for the module prerequisite to be specified using boolean logic, e.g.: requires => q[ (DBD::Pg && DateTime::Format::Pg) || (DBD::mysql && DateTime::Format::mysql) ] If we need to, we can also include version specifications: requires => q[ ( DBD::Pg > 1.1 && DateTime::Format::Pg ) || ( DBD::mysql <= 1.2 && DateTime::Format::mysql ) ] The neat thing about the solution is that after coding it, Marc had PRD generate a parser that ran independantly from PRD itself. This solution, according to the judge, "... is probably the best solution to use if full expression parsing is desired for Module::Build." Marc's outline of his solution and link to full code is available here: http://perl.plover.com/~alias/list.cgi?1:mss:2279:homdmfmmaelddeehiooi =head3 XSH - The XML Editing Shell XSH: L uses Parse::RecDescent =head3 Parsing Symbolic Expressions / Boolean Logic Says Stefan Mueller: Parse::RecDescent can easily parse such expressions. You can find a working example grammar for more involved *arithmetic* expressions in the L module that is part of the L distribution. It should be fairly easy to modify it to work for boolean logic. =head3 MySql to Oracle schema conversion utility Written in Parse::RecDescent by Tim Bunce: L =head2 Other Parsing Engines =head3 py py, which I assume is short for C, is a program by Mark-Jason Dominus which parses GNU Bison output to produce Perl parsers. It is obtainable from http://perl.plover.com/py/ =head3 Parse::YAPP A bottom-up parser which will be familiar to those who have used Lex and Yacc. Parse::RecDescent is a top-down parser. =head3 Parse::Earley L =head3 Perl6::Rules L =head3 perl-byacc L =head3 Domain-specific parsers On CPAN there are many parsers for particular tasks such as CSV, HTML, XML, etc. =head2 a VIM Syntax File for Parse::RecDescent L provides a VIM syntax file for prd. =head1 APPENDIX =head2 Author The author of Parse::RecDescent is Damian Conway. The author of Parse::RecDescent::FAQ is Terrence Brannon . L The (unwitting) contributors to this FAQ =over 4 =item * Chas Owens =item * Brent Dax =item * Damian Conway =item * Gwynn Judd =item * lhoward of Perlmonks =item * Marcel Grunaer =item * Matthew Wickline =item * Randal L. Schwartz (merlyn), Perl hacker =item * Stefan Mueller =item * Yves Orton =back =head2 Sources for FAQ Material I try to regularly scan all sources of Perl question-answer for FAQ material. If you can think of another source that is not listed below, I would appreciate knowing about it. =over 4 =item * comp.lang.perl.modules =item * comp.lang.perl.moderated =item * recdescent@perl.org Conveniently available via NNTP at: news://nntp.perl.org/perl.recdescent =item * perlmonks.org =back =head1 FAQ repo L =cut