Hi bbs2web,

Think of positive/negative look-ahead/look-behind regular expression patterns like this:

Positive Lookahead:
(PATTERN_OF_INTEREST)(?=FOLLOWING_PATTERN)
Capture PATTERN_OF_INTEREST when it precedes the FOLLOWING_PATTERN

e.g.    'this(?=\ that)' # match 'this' when followed by ' that')

Negative Lookahead:
(PATTERN_OF_INTEREST)(?!FOLLOWING_PATTERN)
Capture PATTERN_OF_INTEREST when it DOES NOT precede the FOLLOWING_PATTERN

e.g.    'this(?!\ that)' # match 'this' when NOT followed by ' that'

Positive Look-behind:
(?<=PRECEDING_PATTERN)(PATTERN_OF_INTEREST)
Capture PATTERN_OF_INTEREST when it follows the PRECEDING_PATTERN

e.g.    '(?<=this\ )that' # match 'that' when preceded by 'this '

Negative Look-behind:
(?<!PRECEDING_PATTERN)(PATTERN_OF_INTEREST)
Capture PATTERN_OF_INTEREST when it DOES NOT follow the PRECEDING_PATTERN

e.g.    '(?<!this\ )that' # match 'that' when NOT preceded by 'this '

Both the PRECEDING_PATTERN and the FOLLOWING_PATTERN are matched as the 'look-around' patterns, but neither are captured to a variable, only what is matched as part of your (PATTERN_OF_INTEREST) capture group (i.e. if you use capturing parenthesis).

Edit: replaced 'followed' with 'preceded' as correctly pointed out by AnomalousMonk. Thanks for spotting that :) It was late...

In reply to Re: Remove all lines, except those starting with pattern by perlygapes
in thread Remove all lines, except those starting with pattern by bbs2web

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