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Re^3: Why this Regex code is not working??

by AnomalousMonk (Abbot)
on Sep 23, 2012 at 08:47 UTC ( #995188=note: print w/ replies, xml ) Need Help??


in reply to Re^2: Why this Regex code is not working??
in thread Why this Regex code is not working??

The  use re 'debug'; statement/pragma (see re) is useful to gain insight into what the Perl regex compiler thinks you wrote.

The debug output is just a teensy bit opaque, but some insight can be had just by noting that in both cases below, the  'a' literal substring part of the regex is denoted as
     6:   EXACT <a> (8)
whereas in the case that doesn't work, what you think is a counted quantifier is denoted
    14:   EXACT <{0, 5}> (17)
(i.e., the regex compiler thinks it's a  '{0, 5}' literal substring), versus
    13:   CURLY {0,5} (16)
    15:     REG_ANY (0)
in the case of the correctly written counted quantifier (the one that works as expected).

>perl -wMstrict -le "use re 'debug'; ;; my $rx = qr/(?<WORD>\b\w*a\b)(?<EXTRA>.{0, 5})/; " Compiling REx "(?<WORD>\b\w*a\b)(?<EXTRA>.{0, 5})" Final program: 1: OPEN1 'WORD' (3) 3: BOUND (4) 4: STAR (6) 5: ALNUM (0) 6: EXACT <a> (8) 8: BOUND (9) 9: CLOSE1 'WORD' (11) 11: OPEN2 'EXTRA' (13) 13: REG_ANY (14) 14: EXACT <{0, 5}> (17) 17: CLOSE2 'EXTRA' (19) 19: END (0) floating "{0, 5}" at 2..2147483647 (checking floating) stclass BOUND m +inlen 8 Freeing REx: "(?<WORD>\b\w*a\b)(?<EXTRA>.{0, 5})" >perl -wMstrict -le "use re 'debug'; ;; my $rx = qr/(?<WORD>\b\w*a\b)(?<EXTRA>.{0,5})/; " Compiling REx "(?<WORD>\b\w*a\b)(?<EXTRA>.{0,5})" Final program: 1: OPEN1 'WORD' (3) 3: BOUND (4) 4: STAR (6) 5: ALNUM (0) 6: EXACT <a> (8) 8: BOUND (9) 9: CLOSE1 'WORD' (11) 11: OPEN2 'EXTRA' (13) 13: CURLY {0,5} (16) 15: REG_ANY (0) 16: CLOSE2 'EXTRA' (18) 18: END (0) floating "a" at 0..2147483647 (checking floating) stclass BOUND minlen + 1 Freeing REx: "(?<WORD>\b\w*a\b)(?<EXTRA>.{0,5})"

The YAPE::Regex::Explain module can also offer helpful insight, but unfortunately it doesn't support regex constructs much beyond Perl version 5.6 and that's just what you're using!


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