in reply to Re: substitution in regular expression
in thread substitution in regular expression

Thanks for the idea, I'll write it down. Just one thing, could you explain the: 
(?{print "$1\n"})(?!)

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I don't understand the question mark before the print block. Also why the (?!) at the end. I noticed that removing it only prints ABC, but I don't understand why. Thank you

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Re^3: substitution in regular expression
by trizen (Hermit) on Apr 23, 2014 at 22:52 UTC

    Short explanation: (?{...}) means to execute arbitrary Perl code inside a regular expression, and (?!) makes the regex engine to fail and backtrack, trying to match from the last_pos + 1. When it starts matching ABC, it prints it, fails, backtracks and starts matching from B the next three letters, giving us BCD. The process repeats until the internal regex counter reaches the end of the string.

    I know, I'm really bad at explaining things to humans, but, fortunately, Athanasius explained this better once.

    Please see: Re: RegEx + vs. {1,}

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      Thanks a lot! Very well explained actually. It's a nice trick to know.
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