Regex Search pattern not terminated error

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

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Re: Regex Search pattern not terminated error
by Athanasius (Archbishop) on Mar 12, 2016 at 15:32 UTC

You are right about two things:

the problem expression is \Q:{\E
\Q ... \E (and quotemeta) do indeed escape braces (curly brackets) along with the other regular expression metacharacters.

What’s happening here (and it took me a while to figure it out!) is that the qr expression is also using braces as its delimiter. So the Perl parser first looks for the end of the whole qr{...} expression, and thinks it has found it when it encounters the first right brace; only later does it implement \Q ... \E to backslash non-word characters.

To fix this, either escape the brace manually (\Q:\{\E) or (better) change the delimiter: my $regex = qr! \n ... !mix;

Hope that helps,

Athanasius <°(((>< contra mundum Iustus alius egestas vitae, eros Piratica,

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Re: Regex Search pattern not terminated error
by ww (Archbishop) on Mar 12, 2016 at 15:51 UTC

It appears that Athanasius¹ hit the nail on the head... first shot.

Since my foo didn't provide that insight directly, I tried adding use diagnostics; which produced this illuminating comment:

Search pattern not terminated at D:\_Perl_\PMonks\1157546.pl line 7 (#
+1)
    (F) The lexer couldn't find the final delimiter of a // or m{}
    construct.  Remember that bracketing delimiters count nesting leve
+l.
    Missing the leading $ from a variable $m may cause this error.

    Note that since Perl 5.9.0 a // can also be the defined-or
    construct, not just the empty search pattern.  Therefore code writ
+ten
    in Perl 5.9.0 or later that uses the // as the defined-or can be
    misparsed by pre-5.9.0 Perls as a non-terminated search pattern.

Uncaught exception from user code:
        Search pattern not terminated at D:\_Perl_\PMonks\1157546.pl l
+ine 7.
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The confirmation there of your surmise should lead pretty directly to (somewhat obscure) documentation (that I didn't bother to chase down because you already had a valid answer from the Reverend Canon.

JBTW, my error line as cited by diagnostics is off-by-one because I add a node number to the test code.

Update ¹ ...and choroba who offered similar wisdom as I was preparing this.

Spirit of the Monastery

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Re: Regex Search pattern not terminated error
by choroba (Cardinal) on Mar 12, 2016 at 15:41 UTC

my $regex2 = qr% \n
   (?:
      \QI'll be good.\E
      | Hushing
      | Hrumph
      | \Q>:[\E | \Q:-(\E | \Q:(\E | \Q:-c\E | \Q:c\E | \Q:-<\E | \Q:<
+\E
      | \Q:-[\E | \Q:[\E  | :\{ | \Q:-|\E | \Q:@\E | \Q>:(\E | \Q:'-(\
+E
      | \Q:'(\E
      | \QShutting up now.\E
      | \QBut, but...\E
      | \QI'll be quiet.\E
   )
%mix;
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But, as curly braces are the delimiters, qr{ \{ } turns into qr% { %', , but there's no way how to express 'qr% \{ % with curlies as the delimiters - but you don't need that, as they're equivalent.

That's why I don't like using curlies as regex delimiters.

#!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };

my @res = ( qr% a{1 %,
            qr{ a\{1 },
            qr% a\{1 %,
            qr% a\Q{\E1 %,
            qr{ a\Q\{\E1 },

            qr% a{{1}1 %,
            qr{ a\{{1}1 },
            qr{ a\{{1}1 },
            qr{ a\{\{1\}1 },
            qr% a\{{1}1 %,

          );

say $_, ' a{1 ' =~ $_ for @res;
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Update: more examples in the second code.

($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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