How to remove specific characters in front of another specific character?

Question

In the string:

my_string = 'log   (x)'

I need to remove all spaces ' ' in front of left parentheses '('

This post suggests using:

re.sub(r'.*(', '(', my_string)

which is an overkill because it has the same effect as my_string[my_string.index('('):] and removes also 'log'

Is there some regexpr magic to remove all spaces in front of another specific character?

Are you writing a parser? Consider using a peg instead of regex — fafl, Jan 14 '17 at 12:19
Both rednaw and Jean-Francois' solutions solve my case . Accepting JF's for the non-re based version if that makes sense, let me know if you'd suggest otherwise. — Pythonic, Jan 14 '17 at 12:32
fafl, yes sort of a parser, could you elaborate on the peg, pls? — Pythonic, Jan 14 '17 at 12:35
[Parsing Expression Grammar](https://en.wikipedia.org/wiki/Parsing_expression_grammar) — snakecharmerb, Jan 14 '17 at 12:47
pyPEG should be what you are looking for: https://fdik.org/pyPEG1/#sample — fafl, Jan 14 '17 at 16:08

score 4 · Answer 1 · answered Jan 14 '17 at 12:22

4

Why not just:

re.sub(' +\(', '(', my_string)

answered Jan 14 '17 at 12:22

gitaarik

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Thanks for the answer, see my comment in the original post for selecting which solution to accept. – Pythonic Jan 14 '17 at 12:38

Jean-François Fabre · Accepted Answer · 2017-01-14T12:30:07.270

use forward lookahead:

re.sub(r"\s+(?=\()","",my_string)

the entity between parentheses is not consumed (not replaced) thanks to ?= operator, and \s+ matches any number of whitespace (tab, space, whatever).

And another possibility without regex:

"(".join([x.rstrip() for x in my_string.split("(")])

(split the string according to (, then join it back with the same character applying a rstrip() within a list comprehension)

score 1 · Answer 3 · answered Jan 14 '17 at 12:29

1

You can use a lookahead assertion, see the regular expression syntax in the Python documentation.

import re

my_string = 'log   (x)'
print(re.sub(r'\s+(?=\()', '', my_string))
# log(x)

answered Jan 14 '17 at 12:29

Laurent LAPORTE

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How to remove specific characters in front of another specific character?

3 Answers3