Backreferences in lookbehind

Question

Can you use backreferences in a lookbehind?

Let's say I want to split wherever behind me a character is repeated twice.

    String REGEX1 = "(?<=(.)\\1)"; // DOESN'T WORK!
    String REGEX2 = "(?<=(?=(.)\\1)..)"; // WORKS!

    System.out.println(java.util.Arrays.toString(
        "Bazooka killed the poor aardvark (yummy!)"
        .split(REGEX2)
    )); // prints "[Bazoo, ka kill, ed the poo, r aa, rdvark (yumm, y!)]"

Using REGEX2 (where the backreference is in a lookahead nested inside a lookbehind) works, but REGEX1 gives this error at run-time:

Look-behind group does not have an obvious maximum length near index 8
(?<=(.)\1)
        ^

This sort of make sense, I suppose, because in general the backreference can capture a string of any length (if the regex compiler is a bit smarter, though, it could determine that \1 is (.) in this case, and therefore has a finite length).

So is there a way to use a backreference in a lookbehind?

And if there isn't, can you always work around it using this nested lookahead? Are there other commonly-used techniques?

Interesting, and +1 for your ingenious workaround. I don't use Java, so I can't try it myself - what happens if the backreferenced group is outside the lookaround, like (?<=\\1)(.)? — Tim Pietzcker
@Tim: it results in essentially the same PatternSyntaxException. By the way, if anybody wants to play around with a variant of this problem, I just authored one on codingBat: codingbat.com/prob/p266235 — polygenelubricants
@polygenelubricants I wish I could upvote this regex : (?<=(?=(.)\\1)..) for at least 10 times. very elegant! — Eugene

Tim Pietzcker Tim Pietzcker · Accepted Answer · 2010-04-29T07:59:13

Looks like your suspicion is correct that backreferences generally can't be used in Java lookbehinds. The workaround you proposed makes the finite length of the lookbehind explicit and looks very clever to me.

I was intrigued to find out what Python does with this regex. Python only supports fixed-length lookbehind, not finite-length like Java, but this regex is fixed length. I couldn't use re.split() directly because Python's re.split() never splits on an empty match, but I think I found a bug in re.sub():

>>> r=re.compile("(?<=(.)\\1)")
>>> a=re.sub(r,"|", "Bazooka killed the poor aardvark (yummy!)")
>>> a
'Bazo|oka kil|led the po|or a|ardvark (yum|my!)'

The lookbehind matches between the two duplicate characters!

Backreferences in lookbehind

1 Answers