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in reply to Re: regex not matching special char
in thread regex not matching special char

Interesting. A backslash is a word boundary, but a backslashed dollar sign is itself a non-word char itself, and hence is also a word boundary to the string "AVG" which follows it. Rats!

I need to distinguish between strings such as "$AVG", A$AVG", "A$AVGA". Hence my attempt to do it using \b$AVG\b.

Am I asking too much of Perl regex?

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Re^3: regex not matching special char
by AnomalousMonk (Archbishop) on Dec 14, 2012 at 05:43 UTC

    In addition to looking at the documentation linked by kennethk (and also at perlretut; see in particular the section titled 'Looking ahead and looking behind'), perhaps some insight as to the effect of the  \b (and \B) zero-width word (and non-word) boundary assertions can be gained by split-ting one of the OPed example strings on each assertion: 

    >perl -wMstrict -le
"my $line2 = 'I:\$AVG\hello.log';
 printf qq{'$_'  } for split /\b/, $line2;
 print qq{\n};
 printf qq{'$_'  } for split /\B/, $line2;
"
'I'  ':\$'  'AVG'  '\'  'hello'  '.'  'log'

'I:'  '\'  '$A'  'V'  'G\h'  'e'  'l'  'l'  'o.l'  'o'  'g'

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Re^3: regex not matching special char
by Anonymous Monk on Dec 14, 2012 at 08:50 UTC

    Am I asking too much of Perl regex?

    ;) No, is Perl regex asking too much by asking you to know what you're asking for ? *zing*

    perlrequick

    Luckily YAPE::Regex::Explain handles these 

    use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
'\b\$AVG'
)->explain;
__END__
The regular expression:

(?-imsx:\b\$AVG)

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  AVG                      'AVG'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

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    use YAPE::Regex::Explain;
print YAPE::Regex::Explain->new(
'\w?\$AVG\b'
)->explain;
__END__The regular expression:

(?-imsx:\w?\$AVG\b)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  \w?                      word characters (a-z, A-Z, 0-9, _)
                           (optional (matching the most amount
                           possible))
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  AVG                      'AVG'
----------------------------------------------------------------------
  \b                       the boundary between a word char (\w) and
                           something that is not a word char
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

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      Thanks for the tip on YAPE::Regex::Explain.

      As kennethk pointed out - reading between the lines - the key is to know that a back slashed special character will not be taken as a word character, and hence will be seen as a word boundary itself. That is why the simple \b does not work.

      Armed with a definitive answer as to what is happening, a simple work around can be constructed. In my case the strings I am looking for are actual directory names. I can split on the directory separator.

      Thanks again.
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Re^3: regex not matching special char
by muba (Priest) on Dec 14, 2012 at 04:24 UTC

    What if you change \b to \w?

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