that particular part of Mastering Regular Expressions is out of date and no longer correct. if i recall correctly, MRE is written against 5.004 and lookbehind was added to 5.005.

the lookbehind syntax is as follows:

0-width positive lookbehind assertion: (?<=pattern)
0-width negative lookbehind assertion: (?<!pattern)

however, i think what you want in this case is actually a negative lookahead assertion like the following:

m!^<TR bgcolor=.*>(?!(?:re:\s*)+)([^<]*)</a>!i

also, the 'o' modifier is not very useful in your examples, since there are no variables in the regexp to interpolate.

In reply to Re: lookbehind by mdillon
in thread lookbehind by httptech

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