Regular expression for 'b' not preceded by an odd number of 'a's

Asked Mar 02 '20 at 09:30

Active Mar 02 '20 at 09:47

Viewed 67 times

I want to write a regular expression for a pattern b not preceded by an odd number of repetitions of the letter a using Python's re module. In principle,

(?<=(?<!a)(aa)*)b

would do what I want, but this isn't legal because lookbehinds need to have a fixed length.

How can I do this?

edited Mar 02 '20 at 09:32

VLAZ

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asked Mar 02 '20 at 09:30

schtandard

@WiktorStribiżew Python, apparently. Added the tag for clarity – VLAZ Mar 02 '20 at 09:32
You may use your pattern with Python PyPi `regex` – Wiktor Stribiżew Mar 02 '20 at 09:32
@WiktorStribiżew As I said, I am using Python's `re` module, where they aren't. – schtandard Mar 02 '20 at 09:32
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Since it is a positive lookbehind, you do not even need. Just use `((?<!a)(?:aa)*)b`. Since you capture all `a`s before `b`, you may decide what to do with the match further in the code, or with a backreference in the replacement pattern. – Wiktor Stribiżew Mar 02 '20 at 09:37
I changed the close reason link at the top to showcase this common approach. – Wiktor Stribiżew Mar 02 '20 at 09:48

Regular expression for 'b' not preceded by an odd number of 'a's

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