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I have a hard time to understand the concepts of "lookahead" and "lookbehind". For example, there is a string "aaaaaxbbbbb". If we look at "x", does lookahead mean looking "x" towards "bbbbb" or "aaaaa"? I mean the direction.
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If the regex is
x(?=insert_regex_here)that is a (positive) look*ahead*, which looks ahead, or forwards, in other words towards "bbbb". It means "find an x that is followed byinsert_regex_here".If the regex is
(?<=insert_regex_here)xthat is a (positive) look*behind*, which looks behind, or backwards, in other words towards "aaaa". It means "find an x that is preceded byinsert_regex_here".You can also have negative lookahead
x(?!insert_regex_here)meaning "xnot followed byinsert_regex_here", and negative lookbehind(?<!insert_regex_here)x, meaning "x not preceded byinsert_regex_here".(The above
(?=and(?<!etc are Perl regex syntax - the syntax might be slightly different depending on your flavour of regex).I recommend you read the link that Chad gave in the comments. It has examples.