Python regular expression string substitution

Question

I am interested in finding any occurrence of

(somenumber)

and replacing it with

-somenumber

I have a Perl background and tried this, hoping the (\d+) grouping would populate variable $1:

myterm = re.sub(r"\((\d+)\)", "-\$1",myterm)

However this resulted in a result of the literal

-$1

How to do this in Python?

try r"-\1" instead

aiven
– aiven

2018-01-09 23:01:30 +00:00
Commented Jan 9, 2018 at 23:01 — aiven
– aiven, Commented Jan 9, 2018 at 23:01
Use a raw string: r"-\1" and no $.

cs95
– cs95

2018-01-09 23:01:35 +00:00
Commented Jan 9, 2018 at 23:01 — cs95
– cs95, Commented Jan 9, 2018 at 23:01

ron rothman · Accepted Answer · 2018-01-09 23:55:31Z

1

I see two problems:

You're using Perl's syntax (dollar sign) to dereference the positional match. Python uses \, not $.
Your backslash in "-\$1 is being interpreted by the Python compiler, and is effectively removed before re.sub sees it.

Either using a raw string (as noted in the comments to your question), or escaping the backslash (by double-backslashing), should fix it:

myterm = re.sub(r"\((\d+)\)", r"-\1", myterm)

or

myterm = re.sub(r"\((\d+)\)", "-\\1", myterm)

Tested and confirmed:

import re

myterm = '(1234)'

# OP's attempt:
print re.sub(r"\((\d+)\)", "-\$1", myterm)

# two ways to fix:
print re.sub(r"\((\d+)\)", r"-\1", myterm)
print re.sub(r"\((\d+)\)", "-\\1", myterm)

prints:

-\$1
-1234
-1234

answered Jan 9, 2018 at 23:55

ron rothman

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