python regex question

Question

I want to match certain symbols only when they are not prefixed by specific characters. For instance, match "))))))))))" when it is not preceded by "x". Need some advices. My current expession is

(?<!x|X|:|=|\\1)([\|()\[\]])+

which does not work.

[EDIT] Rephrase my question

@BoltClock, i want to match the items in the captured group like '))))))))' in 's)))))))' when they are not prefixed by the characters in the lookbehind group/ — goh
– goh, Commented Oct 19, 2010 at 10:05
For the ')))' case, the following may do what you need, (?x) (?<! [xX:=)]) ( [][()|]+ ) — a'r
– a'r, Commented Oct 19, 2010 at 10:23

tzot · Accepted Answer · 2010-10-19 17:03:34Z

1

re.search(r"(?<![x)])\)+", text)

>>> re.search(r"(?<![x)])\)+", " hello)))))")
<_sre.SRE_Match object at 0xb75c0c98>
>>> _.group()
')))))'
>>> re.search(r"(?<![x)])\)+", " hellox)))))")
>>>

This makes use of the “negative lookbehind assertion”: we want as many parentheses as possible, not preceded by either "x" or ")" (the latter because otherwise, we would get the parentheses starting from the second parenthesis, preceded by the first parenthesis and therefore not an "x")

answered Oct 19, 2010 at 17:03

tzot

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suzanshakya · Accepted Answer · 2010-10-19 11:04:14Z

1

Use complementing character class: '[^x\)](\)+)'
All your specific characters which should not be prefixed will be placed with x, along with ).

answered Oct 19, 2010 at 11:04

suzanshakya

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