Python regex with named group in conditional expression

Question

I'm having trouble getting a regex to work using a named group and a conditional expression. I've simplified the problem to the smallest possible example.

The regex r"(?P<x>\d)(?(x)a|b)" would seem to mean "match a digit, and if you matched a digit (named group x) also match an a, but if not, match b". This should be equivalent to r"(\da|b)".

However, while it matches for the yes clause, it doesn't match the no clause:

>>> re.match(r"(?P<x>\d)(?(x)a|b)", "5a")
<re.Match object; span=(0, 2), match='5a'>
>>> re.match(r"(?P<x>\d)(?(x)a|b)", "b")
>>>

It's also not working with a numbered group:

>>> re.match(r"(\d)(?(1)a|b)", "5a")
<re.Match object; span=(0, 2), match='5a'>
>>> re.match(r"(\d)(?(1)a|b)", "b")
>>>

What am I missing?

Tom Karzes · Accepted Answer · 2021-06-01 02:18:22Z

1

The problem is the group at the start of the pattern is required to match. You can fix it by making it optional (adding a ? after it):

>>> re.match(r"(?P<x>\d)?(?(x)a|b)", "5a")
<_sre.SRE_Match object; span=(0, 2), match='5a'>
>>> 

>>> re.match(r"(?P<x>\d)?(?(x)a|b)", "b")
<_sre.SRE_Match object; span=(0, 1), match='b'>
>>>

answered Jun 1, 2021 at 2:18

Tom Karzes

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1 Comment

sj95126 Over a year ago

Argh, right. The conditional is never evaluated because b has already failed to match, without the ?. I think I botched my example trying to simplify it. I probably should have asked about the full pattern, but I found another way to do it.

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