using OR operator (|) in variable for regular expression in python

Question

I need to match against a list of string values. I'm using '|'.join() to build a sting that is passed into re.match:

import re
line='GigabitEthernet0/1 is up, line protocol is up'
interfacenames=[
                'Loopback',
                'GigabitEthernet'
                ]
rex="r'" + '|'.join(interfacenames) + "'"
print rex
interface=re.match(rex,line)
print interface

The code result is:

r'Loopback|GigabitEthernet'
None

However if I copy past the string directly into match:

interface=re.match(r'Loopback|GigabitEthernet',line)

It works:

r'Loopback|GigabitEthernet'
<_sre.SRE_Match object at 0x7fcdaf2f4718>

I did try to replace .join with actual "Loopback|GigabitEthernet" in rex and it didn't work either. It looks like the pipe symbol is not treated as operator when passed from string. Any thoughts how to fix it?

the raw prefix is useless in that case. Problem is that your generated regex has simple quotes in it. Remove them: `rex = '|'.join(interfacenames)` — Jean-François Fabre, Jan 12 '17 at 20:41
You didn't copy/paste the same string when you did your test. It would have to be `re.match("r'Loopback|GigabitEthernet'",line)`. — melpomene, Jan 12 '17 at 20:41
Thank you. Was sure I did try it at some point but clearly did something wrong. It works perfectly. — lem, Jan 13 '17 at 00:17

score 2 · Answer 1 · edited May 23 '17 at 11:45

You use the r' prefix as a part of a string literal. This is how it could be used:

rex=r'|'.join(interfacenames)

See the Python demo

If the interfacenames may contain special regex metacharacters, escape the values like this:

rex=r'|'.join([re.escape(x) for x in interfacenames])

Also, if you plan to match the strings not only at the start of the string, use re.search rather than re.match. See What is the difference between Python's re.search and re.match?

score 1 · Answer 2 · answered Jan 12 '17 at 20:41

1

You don't need to put "r'" at the beginning and "'". That's part of the syntax for literal raw strings, it's not part of the string itself.

rex = '|'.join(interfacenames)

answered Jan 12 '17 at 20:41

Barmar

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using OR operator (|) in variable for regular expression in python

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