Parse::RecDescent::FAQ - the official, authorized FAQ for Parse::RecDescent.
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.
Also see the "Left-recursion" section under "PARSER BEHAVIOR"
The variable
$Parse::RecDescent::skip
is what you need. It takes strings and qr-quoted regular expressions. See the Parse::RecDescent docs for details.
qr
Also, see the Terminal Separators parts of the docs.
Terminal Separators
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
Also, regarding elimination see this Perlmonks node:
http://www.perlmonks.org/index.pl?lastnode_id=6364&node_id=153155
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.
Ques: When a subrule in a rule that has a "zero or more" repetition specifier (ie. ? or s?) has a <commit> directive in its production followed by the conditional <error?> <reject> 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 <commit> '[' ']' | <error?> <reject>
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?
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 <commit> '[' ']' | <error?> <reject>
or
myrule: 'stuff' mysubrule(?) /\s*\Z/ mysubrule: ID <commit> '[' ']' | <error?> <reject>
or whatever addition confirms that there really wasn't anything else after 'stuff'.
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 }
Marcel went on to point out an optimization:
another option would be the use of a rulevar:
myrule : <rulevar: local $failed> myrule : 'stuff' mysubrule(?) <reject:$failed> mysubrule: ID <commit> '[' ']' | <error?> { $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.
How do I match the line numbers with the actual contents of my script?
At present, you can't (but that's on the ToDo list). Setting $::RD_TRACE can be useful though:
$::RD_TRACE
Once you've run with $RD_TRACE, do this:
$RD_TRACE
perl -w RD_TRACE
Then go and examine the actual line numbers given for the error in the file RD_TRACE.
RD_TRACE
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).
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.
Answer by Conway
Sure. Treat them as whitespace!
Do something like this:
program: <skip: qr{\s* (/[*] .*? [*]/ \s*)*}x> statement(s) statement: # etc...
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
q{ config: section(s) section: <skip: ''> section_label parameter(s) | <error> section_label: /\[[A-Z]+\]\s*/ parameter: key "=" value key: /[A-Za-z0-9 ]+/ value: /.*\n/ }
Another Answer
Also take a look at the example in section 11.2 in "Data Munging with Perl" by Dave Cross.
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?
By default, P::RD skips all whitespace (*including* newlines) before tokens.
Try this instead:
line: <skip: qr/[ \t]*/> word(s) newline { print "Found a line\n"; } word: /\w+/ newline: /\n/
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)
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 <reject: length $1 != 47> { @{$return}{title major minor)} = ($2,$3,$4) }
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".
A simple answer that is wrong:
Hmm, I guess I could make this simpler and do this:
line: word <reject: $thiscolumn != 22> word word: /\S+/
right?
Wrong. And the reason why is that The
<reject:...>
won't skip any whitespace after the first word.
You instead would want:
line: word '' <reject: $thiscolumn != 22> word
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 0 from evaluating this conditional, but unfortunately, that does not lead to failure.
0
On the other hand, <reject> 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
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 (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:
(x,y)
macro: identifier ':' expr
where, again, expr is defined fully in the grammar. Adding arguments to this... I need to instantiate my (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.
And the answer is...
http://perlmonks.org/index.pl?node_id=176399
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.
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");
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
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?
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); } [...] }
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 | <error> def : <rulevar: local $p_cons> def : <rulevar: local $p_text> # 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 : <rulevar: local $m_cons> meaning : // { $m_cons = consumer($text) } element(s /,?/) { $m_cons->($text) } element : alternation(s /\|/) alternation : expr(s /[+>]/) expr : /!?/ term term : ident '(' meaning ')' | ident
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 <reject> directive to simulate such a behaviour.
Is there any easy solution to this?
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;
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: <matchrule: $arg[0]>(s) { my %seen; foreach (@{$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($parser->line($_)); }
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"
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 QuotedText, Comma, and you're done!
QuotedText
Comma
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.
Take a look at Parse::RecDescent's precompilation option under the section titled "Precompiling parsers".
Reduce the "depth" of the grammar. Use fewer levels of nested subrules.
Where possible, use regex terminals instead of subrules. For example, instead of:
string: '"' char(s?) '"' char: /[^"\\]/ | '\\"' | '\\\\'
write:
string: /"([^"\\]|\\["\\])*"/
Where possible, use string terminals instead of regexes.
Use repetitions or <leftop>/<rightop> instead of recursion.
For example, instead of:
list: '(' elems ')' elems: elem ',' elems | elem
list: '(' <leftop: elem ',' elem> ')'
or:
list: '(' elem(s /,/) ')'
Factor out common prefixes in a set of alternate productions.
id: name rank serial_num | name rank hair_colour | name rank shoe_size
id: name rank (serial_num | haircolour | shoe_size)
Pre-parse the input somehow to break it into smaller sections. Parse each section separately.
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).
Consider whether you would be better porting your grammar to Parse::Yapp instead.
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 @item will have one TextChar from position 2 until all TextChars are matched. However, the true structure of @item is:
@item
TextChar
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]} ) }
I can't seem to get the text from my subrule matches...
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)
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?
To turn off whitespace skipping so I can handle it manually, I always use:
<skip:''>
See:
demo_decomment.pl demo_embedding.pl demo_textgen.pl
for examples.
What's wrong?!
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 | <error>
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.
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, rule_one returns a scalar upon matching, while rule_two returns an arrayref with 1 element upon matching:
rule_one
rule_two
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 x is the right analogy for RecDescent repetition operators. The behaviour of * and + in regexes is a closer model.
x
*
+
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.
resync 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.
resync
For example:
range: '(' number '..' number )' { $return = $item{number} }
will return only the value corresponding to the last match of the number subrule.
number
To get each value for the number subrule, you have a couple of choices, both documented in the Parse::RecDescent manpage under @item and %item.
@item and %item
If you say:
somerule: <rulevar: $x>
you get a lexical $x within the rule (only). If you say:
somerule: <rulevar: local $x>
you get a localized $x within the rule (and any subrules it calls).
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.
See Commit in subrule which is optional in rule
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 ??
Like this:
use Parse::RecDescent; open (Parse::RecDescent::ERROR, ">errfile") or die "Can't redirect errors to file 'errfile'"; # your program here
What I want to do is export a <error: foo bar> 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.
You can get access to the error data by referring to the attribute $thisparser-{errors}> within a rule.
$thisparser-
$thisparser-{errors}> is a reference to an array of arrays. Each of the inner arrays has two elements: the error message, and the line number.
So, for example, if your top-level rule is:
start: subrule1 subrule2
then you could intercept any errors and print them to a log file, like so:
start: subrule1 subrule2 | { foreach (@{$thisparser->{errors}}) { print LOGFILE "Line $_->[1]:$_->[0]\n"; } $thisparser->{errors} = undef; }
Note that the line:
$thisparser->{errors} = undef;
is doing double duty. By resetting $thisparser-{errors}>, it's preventing those annoying error messages from being automagically printed. And by having the value undef 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 undef.
undef
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' <commit> FactionID String Command:'attack' <commit> 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) | <error> ... Any better solutions?
You can just do:
Command: '#atlantis' <commit> FactionID String | 'attack' <commit> Number(s) | <error>
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 <error?>, which will only print the error when it saw '#atlantis' or 'attack' (because then you are committed).
Is there a way to to collect the parser-generated errors and use them later on in my script?
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.
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.
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 '', <DATA>) 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]} }
Example: a, (b ,c, (e,f , [h, i], j) )
Answer by FAQ author
Maybe Text::Balanced is enough for your needs. See it on search.CPAN.org under author id DCONWAY.
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: <leftop: item ',' item> item: word '(' list ')' <commit> | word word: /\w+/ }; my $parser = new Parse::RecDescent ($grammar) or die "Bad grammar!\n"; defined $parser->list($teststr) or print "Bad text!\n";
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?
Use a scored grammar. For example, this scoring directive
opcode: /$match_text1/ <score: { length join '' @item}> opcode: /$match_text2/ <score: { length join '' @item}> opcode: /$match_text3/ <score: { length join '' @item}>
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.
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 : <skip:''> '%' 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
<skip:''> '%' 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 <skip:''>'%' file production
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:
'%' <skip:''> file
in your rule. back
How do I match an arbitrary number of blank lines in Parse::RecDescent?
Unless you use the /m suffix, the trailing $ means "end of string", not "end of line". You want:
blank_line: /^\s+?$/m
blank_line: /^\s+?\n/
I have a rule which MUST be failing, but it isn't. Why?
blank_line: { $text =~ /silly-regex/ } parses with no error.
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).
See the documentation for the $text variable.
$text
my $grammar = <<EOGRAMMAR; 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"; } | <error> EOGRAMMAR Why does '@' have to be escaped? And whatever reason that may be, why doesnt it apply to '\n'?
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"; } | <error> EOGRAMMAR
Such-and-such a module works fine when I don't use Parse::RecDescent
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 $/; }
What is the Perl idiom for getting the leftmost shortest match? For instance, so:
$a = "banana"; $a =~ /b.*n/; # but different print $&;
would yield "ban" instead of "banan"?
$a =~ /b.*?n/;
I'm playing around with the <skip:> directive and I've noticed something interesting that I can't explain to myself.
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 } } | <error> number: <skip: '\.*'> /\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:
<skip: "\.*">
the grammar fails to parse the input. However, if I put square brackets around the escaped dot:
<skip: "[\.]*">
the grammar starts working again:
How does this work this way?
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:
<skip: "\\.*">
The reason the third variant:
works is because it becomes the pattern:
/[.]/
which is a literal dot.
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 : <rulevar: local $rule_text> report : <rulevar: local $rule_offset> 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?
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 $text and evaluates them later to get the text consumed.
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.
Written by "Philip 'Yes, that's my address' Newton" <nospam.newton@gmx.li>
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 <nospam.newton@gmx.li> If you're not part of the solution, you're part of the precipitate
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?
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 $_[0] into a list (of arrayrefs) # turn each (arrayref) element of that list # into an anonymous array containing # a list found by derefrencing the arrarref
Try plugging that line into above script instead of the line from the demo_calc.pl and you'll see different output. The line from demo_calc.pl is in fact doing extra useless work. My guess is that the extra @{[ ]} around there is one of two things: 1) a momentary lapse of attention resulting in a copy/paste error, or duplicate typing or 2) an artifact of earlier code wherein something extra was going on in there and has since been deleted.
Even Damian can make a mistake, but it's not a mistake that affects output... it just makes for a tiny bit of wasted work (or maybe Perl is smart enough to optimze away the wasted work, I dunno).
* Damian Conway says:
I have no recollection of why I did this (see children, that's why you should *always* comment your code!).
I *suspect* it's vestigal -- from a time when contents of the argument array reference were somehow modified in situ, but it was important that the original argument's contents not be changed.
The ungainly @{[@{$_[0]}]} syntax is a way of (shallow) copying the array referenced in $_[0] without declaring a new variable. So another possible explanation is that evalop may originally have been a one-liner, in which case I might have used this "inlined copy" to keep the subroutine's body to a single expression.
@{[@{$_[0]}]}
However...
Even Damian can make a mistake
is by far the likeliest explanation.
MySql to Oracle schema conversion utility
Written in Parse::RecDescent by Tim Bunce:
http://groups.google.com/groups?q=recdescent&start=40&hl=en&scoring=d&rnum=41&selm=9km7b1%246vc%241%40FreeBSD.csie.NCTU.edu.tw
Data::MultiValuedHash on search.cpan.org
Transparent manipulation of single or multiply-valued Perl hash values.
Parse::Recdescent tutorial at www.perl.com
http://www.perl.com/pub/a/2001/06/13/recdecent.html
"Safe undumping"
In this Linux Magazine article by Randal Schwartz uses Parse::RecDescent to parse Data::Dumper output. Not fast, but quite complete.
"Parsing Interesting Things", SysAdminMag.COM, December 2001, Randal Schwartz
py
py, which I assume is short for parse yacc, is a program by Mark-Jason Dominus which parses GNU Bison output to produce Perl parsers.
parse yacc
It is obtainable from http://perl.plover.com/py/
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.
Text::Balanced
Use this instead of writing hairy regular expressions to match certain common "balanced" forms of text, such as tags and parenthesized text.
"Mastering Regular Expressions" by Jeffrey Freidl
You still need to know when to use /.*/ or /.+/ or /[^x]*/
"Data Munging with Perl" by Dave Cross
Chapter 11 is focused on parsing with special emphasis on practical use of Parse::RecDescent.
"Object-Oriented Perl" by Damian Conway
This book will aid you in complexity management for large grammars.
http://www.PerlMonks.org
A useful site to get fast help on Perl.
The author of Parse::RecDescent::FAQ is Terrence Brannon <tbone@cpan.org>.
The author of Parse::RecDescent is Damian Conway.
The (unwitting) contributors to this FAQ
Me
Damian Conway
Marcel Grunaer
Brent Dax
Randal L. Schwartz (merlyn), Perl hacker
lhoward of Perlmonks
Matthew Wickline
Gwynn Judd
7 POD Errors
The following errors were encountered while parsing the POD:
=over should be: '=over' or '=over positive_number'
=back doesn't take any parameters, but you said =back =head2 More on blank lines
You can't have =items (as at line 1970) unless the first thing after the =over is an =item
Expected '=item *'
To install Parse::RecDescent::FAQ, copy and paste the appropriate command in to your terminal.
cpanm
cpanm Parse::RecDescent::FAQ
CPAN shell
perl -MCPAN -e shell install Parse::RecDescent::FAQ
For more information on module installation, please visit the detailed CPAN module installation guide.