Java regex error - Look-behind with group reference

7
votes

I'm trying to build a regex that matches exactly two occurrences of a char in a class. This is the regex I made:

(?<!\1)([^raol1c])\1(?!\1)

As you can see, it uses negatives look-aheads and behind. But, as usual the latter does not work; java throws the well-known exception "look-behind group does not have an obvious maximum length" when it clearly has a maximum length (exactly one char).

Ideally the regex should match "hh", "jhh", "ahh", "hhj", "hha" but not "hhh".

Any ideas about how to deal with this and make a workaround?

1
javaregexbackreferencenegative-lookaheadnegative-lookbehind
Is there a set length for the strings you are checking? - Andrew Clark
No, strings could be of any length. - user2402372
I don't understand your rule: should "hhaa" match? - jlordo
Yes. Because the regex match "hh" then look ahead for a negative match of "h" and looks behind for a negative match of "h" what is true in both cases. - user2402372
For a really simple workaround beyond a single regex, search for ([^raol1c])\1+ and check that the result is not more than two characters long. - Martin Ender

1 Answers

6
votes

Here is a workaround. It's ugly but apparently it works:

(?<!(?=\1).)([^raol1c])\1(?!\1)

Putting the backreference into a zero-length lookahead inside the lookbehind makes the lookbehind certainly of fixed length.

Disclaimer, I did not come up with this (unfortunately): Backreferences in lookbehind

EDIT:

The above pattern does not rule out hhh for some reason. However, this works:

(?<!(.)(?=\1))([^raol1c])\2(?!\2)

If we create the first group inside the lookbehind then we can use this to ensure that the first character after the lookbehind is not the same as the one before it.

Working demo.