Why doesn't finite repetition in lookbehind work in some flavors?

Question

I want to parse the 2 digits in the middle from a date in dd/mm/yy format but also allowing single digits for day and month.

This is what I came up with

Answer 1

In addition to those listed by @polygenelubricants, there are two more exceptions to the "fixed length only" rule. In PCRE (the regex engine for PHP, Apache, et al) and Oniguruma (Ruby 1.9, Textmate), a lookbehind may consist of an alternation in which each alternative may match a different number of characters, as long as the length of each alternative is fixed. For example:

(?<=\b\d\d/|\b\d/)\d{1,2}(?=/\d{2}\b)

Note that the alternation has to be at the top level of the lookbehind subexpression. You might, like me, be tempted to factor out the common elements, like this:

(?<=\b(?:\d\d/|\d)/)\d{1,2}(?=/\d{2}\b)

...but it wouldn't work; at the top level, the subexpression now consists of a single alternative with a non-fixed length.

The second exception is much more useful: \K, supported by Perl and PCRE. It effectively means "pretend the match really started here." Whatever appears before it in the regex is treated as a positive lookbehind. As with .NET lookbehinds, there are no restrictions; whatever can appear in a normal regex can be used before the \K.

\b\d{1,2}/\K\d{1,2}(?=/\d{2}\b)

But most of the time, when someone has a problem with lookbehinds, it turns out they shouldn't even be using them. As @insin pointed out, this problem can be solved much more easily by using a capturing group.

EDIT: Almost forgot JGSoft, the regex flavor used by EditPad Pro and PowerGrep; like .NET, it has completely unrestricted lookbehinds, positive and negative.