Re: Using negative lookahead

[LanX]

> I mostly want to understand why a negative lookahead isn't working the way I thought it would.

The negative lookahead (.*(?!\g1)) says

• "Fail if I can match the delimiter".

And since all your examples end with an delimiter or even never start with one you have not one match.

I think this (.*(?!\g1).) ((?!.*\g1.)) should fix it

• "Fail if I can match the delimiter which is still followed by any other character"

edit

personally I would advice to use a negated character class. (Update see here )

update

that's what you want?

(update: nope its still wrong, am too tired and tybald89 already got it right :)

'abc"def'
 1- Got a match: delimiter was {'}, body was {abc"def}
 2- No match.

'abc'
 1- Got a match: delimiter was {'}, body was {abc}
 2- No match.

"abc"
 1- Got a match: delimiter was {"}, body was {abc}
 2- No match.

''
 1- Got a match: delimiter was {'}, body was {}
 2- Got a match: delimiter was {'}, body was {}

'abc'def'
 1- Got a match: delimiter was {'}, body was {abc'def}
 2- No match.

'This shouldn't match'
 1- Got a match: delimiter was {'}, body was {This shouldn't match}
 2- No match.

"This isn't a problem"
 1- Got a match: delimiter was {"}, body was {This isn't a problem}
 2- No match.

"abc
 1- No match.
 2- No match.

abc"
 1- No match.
 2- No match.

abc
 1- No match.
 2- No match.

'abc"
 1- No match.
 2- No match.

'ab''
 1- Got a match: delimiter was {'}, body was {ab'}
 2- No match.

#!/usr/bin/env perl
use warnings;
use strict;

my @cases = (
    q{'abc"def'},
    q{'abc'},
    q{"abc"},
    q{''},
    q{'abc'def'},               # Want this to fail matching
    q{'This shouldn't match'},  # Want this to fail matching
    q{"This isn't a problem"},
    q{"abc},
    q{abc"},
    q{abc},
    q{'abc"},
    q{'ab''},  # Want this to fail matching
);

strip_quotes($_) for @cases;

# If we can remove a matching pair of single or double quotes from 
# a string, without the quote symbol also appearing within the string,
# do so. Otherwise don't change the string.

sub strip_quotes {
    my $line = shift;
    print "\n$line\n";

    # NO NEGATIVE LOOKAHEAD
    # This works except it allows an embedded delimiter
    if ( $line =~ m{^         # anchor
                    (         # capture delimiter in pos 1
                        ["']  # delim is single or double quote
                    )
                    (.*)      # anything
                    \g1$}x    # finally, the delim
                ) {
        print " 1- Got a match: delimiter was {$1}, body was {$2}\n";
    }
    else {
        print " 1- No match.\n";
    }

    # ATTEMPTING NEGATIVE LOOKAHEAD
    # This should fail if the delimiter is found in non-terminal pos.
    if ( $line =~ m{^         # anchor
                    (         # capture delimiter in pos 1
                        ["']  # delim is single or double quote
                    )
                    ((?!.*\g1.))  # neg lookahead for delim
                    \g1$}x    # finally, the delim
                ) {
        print " 2- Got a match: delimiter was {$1}, body was {$2}\n";
    }
    else {
        print " 2- No match.\n";
    }

}

[ibm1620]

Thank you for "Fail if I can match the delimiter which is still followed by any other character".

I would have used a negated character class except I wanted to maybe support multi-character quotes (e.g. two apostrophes, ''). But mostly I just wanted to understand the negative lookahead!