[Python-Dev] Finding overlapping matches with re assertions: bug or feature? (original) (raw)

Tim Peters tim.peters at gmail.com
Fri Nov 15 07:48:33 CET 2013

I was surprised to find that "this works": if you want to find all overlapping matches for a regexp R, wrap it in

 (?=(R))

and feed it to (say) finditer. Here's a very simple example, finding all overlapping occurrences of "xx":

pat = re.compile("(?=(xx))")
for it in pat.finditer("xxxx"):
    print(it.span(1))

That displays:

(0, 2)
(1, 3)
(2, 4)

Is that a feature? Or an accident? It's very surprising to find a non-empty match inside an empty match (the outermost lookahead assertion). If it's intended behavior, it's just in time for the holiday season; e.g., to generate ASCII art for half an upside-down Christmas tree:

pat = re.compile("(?=(x+))")
for it in pat.finditer("xxxxxxxxxx"):
    print(it.group(1))

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