Java regex look-behind group does not have obvious maximum length error

Question

I know that java regex does not support varying length look-behinds, and that the following should cause an error

(?<=(not exceeding|no((\\\\w|\\\\s)*)mor         


        

Answer 1

java regex does not support varying length look-behinds

It is not totally true, Java supports limited variable length lookbehinds, example (?<=.{0,1000}) is allowed or something like (?<=ab?)c or (?<=abc|defgh).

But if there is no limit at all, Java doesn't support it.

So, what is not obvious for the java regex engine for a lookbehind subpattern:

a {m,n} quantifier applyed to a non-fixed length subpattern:

(?:abc){0,1} is allowed

(?:ab?)?     is allowed
(?:ab|de)    is allowed
(?:ab|de)?   is allowed

(?:ab?){0,1}   is not allowed
(?:ab|de){1}   is not allowed
(?:ab|de){0,1} is not allowed # in my opinion, it is because of the alternation.
                              # When an alternation is detected, the analysis
                              # stops immediatly

To obtain this error message in this particular kind of cases, you need two criterae:

• a potentially variable length subpattern (ie: that contains a quantifier, an alternation or a backreference)

• and a {m,n} type quantifier.

All these cases don't seem evident for the user, since it seems like an arbitrary choice. However, I think that the real reason is to limit the pre-analysis time of the pattern by the regex engine transmission.