sathiya.sw has asked for the wisdom of the Perl Monks concerning the following question:

can some body explain me, how does the following assigns the data to the variable correctly. 
$_ = 'ab$=$12a3$=$45$=$4thfield$=$5th';

( $a, undef , $b ) = $_ =~ /(\w+)\$=\$(\w+)\$=\$(\w+)/;

print "=>$a<==>$b<=\n";

[download]
My question is:
1. How does it only stores the captured things to the variable, why can't it store the $=$ into the variable ?
2. Is it internally mentioned that it should store the $1, $2, $3 ?? or any other mechanism ?
Sathiyamoorthy

Replies are listed 'Best First'.
Re: regex; how it works
by ikegami (Patriarch) on Feb 28, 2009 at 09:11 UTC

    The match operator in list context returns the captures. This is actually documented in perlop. (Not in perlre, which documents regexp patterns.)

    why can't it store the $=$ into the variable ?

    What makes you think it can't? If you're not capturing the right thing, move the parens to capture the right thing.

[reply]
      This clarifies me "The match operator in list context returns the captures." .. Thanks.
      Sathiyamoorthy
[reply]
Re: regex; how it works
by codeacrobat (Chaplain) on Feb 28, 2009 at 17:49 UTC
    See YAPE::Regex::Explain 
    The regular expression:

(?-imsx:(\w+)\$=\$(\w+)\$=\$(\w+))

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):
----------------------------------------------------------------------
  (                        group and capture to \1:
----------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  =                        '='
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  (                        group and capture to \2:
----------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \2
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  =                        '='
----------------------------------------------------------------------
  \$                       '$'
----------------------------------------------------------------------
  (                        group and capture to \3:
----------------------------------------------------------------------
    \w+                      word characters (a-z, A-Z, 0-9, _) (1 or
                             more times (matching the most amount
                             possible))
----------------------------------------------------------------------
  )                        end of \3
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

    [download]
    print+qq(\L@{[ref\&@]}@{['@'x7^'!#2/"!4']});
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Re: regex; how it works
by locked_user sundialsvc4 (Abbot) on Feb 28, 2009 at 16:42 UTC

    The matched regular-expression, which contains parenthesized groups to “capture” various matching-parts out of the input string, returns those matches as a list. The construct ( $a, undef , $b ) = ... is a list assignment, albeit a faintly-unusual one due to the presence of undef as the second term. The corresponding elements of the returned list are assigned as-shown.

    BTW, this is why you can swap two variables so easily:   ($a,$b) = ($b,$a); ... presto!

Re: regex; how it works
by Corion (Patriarch) on Feb 28, 2009 at 08:59 UTC
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