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Perl: the Markov chain saw

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(Unfortunately, the straightforward attempt  $str =~ s/[+-](\d+).{\1}//g; doesn't work.)
How come the straight forward way doesn't work?

Because the regex compiler will attempt to compile the entire  [+-](\d+).{\1} regex (the 'search' regex of the substitution) at compile time, but the  \1 backreference of the  .{\1} counted quantifier sub-expression is not known until run time, when something may be captured that it can actually refer back to. OTOH, the  (??{".{$1}"}) 'postponed' extended pattern is specifically designed to both compile and run at run-time.

BTW: The use of  $^N is, IMHO, 'safer' than the use of  $1 in the sub-expression  (??{ ".{$1}" }) (making it  (??{ ".{$^N}" }) instead) because  $^N equals the contents of the most recently closed capturing group and will not change (semantically) if the relative positional relationship between that capture group and the use of  $^N does not change; whereas adding another capture group anywhere before the  (\d+) group will change the semantics of  $1 because capture group counting will change.

In reply to Re^3: Find Number in String then Ignore Characters proceeding by AnomalousMonk
in thread Find Number in String then Ignore Characters proceeding by BenPen95

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