nathaniels has asked for the wisdom of the Perl Monks concerning the following question:

I must be missing something obvious in here but I just can't seem to find it. Trying to craft a grammar using Regexp::Grammars and I can't get this code to compile. I keep getting the "Quantifier follows nothing" error. 

 my $token_grammar = qr {
        
        <nocontext:>
        <debug: on>
        
        <[unit]>+
        
        <token: unit>                  <word> | <punctuation>
        <token: word>             \b \w+ \b   |  <hyphenword>  |  <con
+traction>
        <token: hyphenword>  \b \w+ ( - | -- ) \w+ \b
        <token: contraction>    \b \w+ ' \w \b
        <token: punctuation>   ( \.+ | ,+ | ?+ | !+ | "+ | '+ | ;+ | :
++ | -+ | &+ | \(+ | \)+ | \[+ | \]+ )
        
    };

[download]

I've tried reducing it to just

 
my $token_grammar = qr {
        
        <nocontext:>
        <debug: on>
        
        <[unit]>+
        
        <token: unit>                  <word>
        <token: word>             \b \w+ \b  
}

[download]

But this just gives be a "Nested Quantifiers" error. Any ideas? Thanks for looking!

Replies are listed 'Best First'.
Re: Quantifier follows nothing error in Regexp::Grammars (quantifier "?")
by Anonymous Monk on Oct 09, 2013 at 06:57 UTC

    The question mark is a quantifier 

    $ perl -le " qr{\d****} "
Nested quantifiers in regex; marked by <-- HERE in m/\d** <-- HERE **/
+ at -e line 1.

$ perl -le " qr{(?#an error follows)****} "
Quantifier follows nothing in regex; marked by <-- HERE in m/(?#an err
+or follows)* <-- HERE ***/ at -e line 1.

$ perl -le " qr{?} "
Quantifier follows nothing in regex; marked by <-- HERE in m/? <-- HER
+E / at -e line 1.

    [download]

    You need to learn to character class :) See perlrecharclass

    Try  <token: punctuation>  ( [.,?!"';:\-&()\[\]]+ )

    See also RFC: A walkthrough from JSON ABNF to Regexp::Grammars and The usage of <ALIAS=\IDENT> of Regexp::Grammars

[reply]
[d/l]
[select]

      Minor comment: the hyphen should be the first character in the character class: 

      <token: punctuation>  ( [-.,?!"';:\&()\[\]]+ )

      [download]

      otherwise it is interpreted as defining a range rather than a literal hypen.

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[d/l]

        the hyphen should be the first character in the character class ... otherwise it is interpreted as defining a range rather than a literal hypen.

        ...or the last...
        ...or escaped with a backslash \.

        Cheers, Sören

        Créateur des bugs mobiles - let loose once, run everywhere.
        (hooked on the Perl Programming language)

[reply]
[d/l]
        perldoc perlrecharclass says you don't need to escape [ within a (non-Posix) character class, and for ] there is a similiar rule as for the hyphen, so 
        <token: punctuation>  ( [][.,?!"';:&()-]+ )

        [download]
        should be valid without any backslash.
        Update: corrected position of hyphen, had it outside of the regex :-/
[reply]
[d/l]
[select]

        Minor comment: the hyphen should be the first character in the character class:

        Why do you think so, what are the reasons for this habit/style?

        I vaguely recall maybe that I read this advice before, but it didn't stick :) (not memorable)

[reply]
Re: Quantifier follows nothing error in Regexp::Grammars
by hdb (Monsignor) on Oct 09, 2013 at 06:10 UTC

    Could you please post a complete script that exhibits your error?

    Update: I think you also need a /x modifier like my $token_grammar = qr {...}x;.

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[d/l]

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