How to know which regex patterns from multiple used matched string in Python

Question

I'm matching multiple regex patterns vs very longs strings (up to 10e8 characters). Is there a way to know which of my regex patterns report match in Python? Or should I rather do regex separately for each pattern?

pat=re.compile('C[GT]GG|A[AT]TA|T[TG]TA')
for m in pat.finditer(longString):
  print m.start(), m.end()
  # how to know which pat matched?

anubhava · Accepted Answer · 2015-09-09 14:31:08Z

1

You can use

m.group()

to see which part of regex matched your input.

>> for m in pat.finditer('pat290'):
...     print m.start(), m.end(), m.group()
...
0 4 pat2

>>> for m in pat.finditer('pat3789'):
...     print m.start(), m.end(), m.group()
...
0 4 pat3

>>> for m in pat.finditer('some-pat1234567'):
...     print m.start(), m.end(), m.group()
...
5 9 pat1

answered Sep 9, 2015 at 14:31

anubhava

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2 Comments

Leszek Over a year ago

thanks anubhava, actually I know about m.group(), but my patterns are a bit more complicated than 'pat1', it maybe something more like 'A[AC][GC]T'. I have specified it in the question now.

anubhava Over a year ago

Hmm I don't think you can get C[GT]GG from matched results.

davidedb · Accepted Answer · 2015-09-10 10:24:26Z

You can use the lastindex (or lastgroup) attribute of re match objects, which reports the index (or group name) of the last matched group.

However you have to modify your regular expression in order to transform each subexpression into a group, by enclosing them between brackets:

pat=re.compile('(C[GT]GG)|(A[AT]TA)|(T[TG]TA)')
for m in pat.finditer(longString):
    print m.start(), m.end(), 'group index:', m.lastindex

If you like to use symbolic names (thus improving readability), the pattern syntax is a little more complicated:

pat=re.compile('(?P<C_CG_GG>C[GT]GG)|(?P<A_AT_TA>A[AT]TA)|(?P<T_TG_TA>T[TG]TA)')
for m in pat.finditer(longString):
    print m.start(), m.end(), 'group index:', m.lastindex, 'group name:', m.lastgroup

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