CODEWITHSUNDEEP
pythonregex
Mark Ginsburg
Mark Ginsburg

Reputation: 2269

Python regular expression string substitution

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?

Upvotes: 1

Views: 65

Answers (1)

ron rothman
ron rothman

Reputation: 18148

I see two problems:

  1. You're using Perl's syntax (dollar sign) to dereference the positional match. Python uses \, not $.

  2. 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

Upvotes: 1

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