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... how to parse a string which has repetitive data ...

As choroba pointed out, every  (pattern) pair of parentheses in a regex captures something (even undef possibly) to its corresponding capture variable. One way to parse a string using nested regexes is avoid using a gazillion capturing groups. Use the non-capturing  (?:pattern) instead for grouping. See perlre, perlrequick, perlretut. In the IP example (but this should generalize to any repetitive data you wish to extract):

>perl -wMstrict -le "my $decimal_octet = qr{ 2 (?: [0-4] \d | 5 [0-5]) | [01]? \d? \d }xms; my $ip = qr{ (?<! \d) $decimal_octet (?: \. $decimal_octet){3} (?! \d) }xms; print $ip; ;; my $s = ' xx yyy zz'; my @ips = $s =~ m{ $ip }xmsg; printf qq{'$_' } for @ips; " (?^msx: (?<! \d) (?^msx: 2 (?: [0-4] \d | 5 [0-5]) | [01]? \d? \d ) (? +: \. (?^msx: 2 (?: [0-4] \d | 5 [0-5]) | [01]? \d? \d )){3} (?! \d) ) '' ''

Note that neither  (?:pattern) nor the  (?<!pattern) (?!pattern) look-around assertions capture. Indeed, nothing captures (to a capture variable) since data is extracted in list context directly to an array.

In reply to Re^3: Using variable to hold regex expression by AnomalousMonk
in thread Using variable to hold regex expression by salatconed

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