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Re^3: regex: negative lookahead

by kennethk (Abbot)
on Dec 05, 2011 at 16:38 UTC ( #941894=note: print w/replies, xml ) Need Help??


in reply to Re^2: regex: negative lookahead
in thread regex: negative lookahead

The difference is a lookahead versus a lookbehind. If you ignore the lookahead/lookbehind because they are zero width (don't consume characters), you would expect /^($start(\w+(,\s)?)+)$/ to grab an entire string between start and finish. When you get to the part of the regular expression where you have your assertion, the cursor is here:
log4j.rootLogger=INFO, FILE, SYSLOG ^
If you look ahead of this position, there is nothing, which clearly passes the negative lookahead check. The offending string is behind your position, thus you need a negative lookbehind.

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[Corion]: choroba: Yeah, that's why I like that approach; it seems even quite obvious in retrospect to me, but many things are in hindsight ;)

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