Re^3: Find Number in String then Ignore Characters proceeding

(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.

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Re^4: Find Number in String then Ignore Characters proceeding by Eliya (Vicar) on May 29, 2012 at 23:44 UTC
...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 I think it's worth pointing out that in other cases this works just fine. For example, in `/(\d+) \1/`, a placeholder for the captured value is compiled into the regex. I.e., the following will match `print "123 123" =~ /(\d+) \1/ ? "matched" : "didn't match";` [download] It's just that with more complex patterns, it doesn't seem to be implemented. (I don't see any compelling reason why - in principle - the count in question here couldn't just as well be compiled to refer to a captured value.)	[reply] [d/l] [select]

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Re^4: Find Number in String then Ignore Characters proceeding
by Eliya (Vicar) on May 29, 2012 at 23:44 UTC

...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

I think it's worth pointing out that in other cases this works just fine. For example, in /(\d+) \1/, a placeholder for the captured value is compiled into the regex. I.e., the following will match

print "123 123" =~ /(\d+) \1/ ? "matched" : "didn't match";
[download]

It's just that with more complex patterns, it doesn't seem to be implemented. (I don't see any compelling reason why - in principle - the count in question here couldn't just as well be compiled to refer to a captured value.)

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