pcre - What's the technical reason for "lookbehind assertion MUST be fixed length" in regex? -

For example, the cause of the regex reporting failure is below The claim of the look-on is unclear. :

  (? :(?: Src) | (?: Href)) = ["\ ']?) ((?:? Https? FTP]: // [^ \ S \ '" ()] +) #s  

for such restriction lookahead not available.

Lookahead and lookbehind are not nearly identical to their names. The letterhead expression is exactly the same as it was a standalone reggae, except that it is anchored in the current match situation and it does not consume it.

Likhebind is a whole different story, starting with the current match situation, it goes back through a letter of text at one go, trying to achieve its expression in every situation. In cases where a match is not possible, before looking at it at the beginning of the text (remembering one letter at a time, remembering) is to go all the way, compare it with Lookahahad expression, which once Applies properly.

This is a big simplification, certainly not, and not all the flavors work in this way, but you get this idea. The way in which it is searched is basically different (and many, less efficient than ) the way the Lookahs are applied. It only makes sense to see how far in the direction of seeing.