Python regex replace capture

Question

I'm trying to add one space before and after +-, using re.sub (only) what expression to use?

import re

text = """a+b
a+b-c
a + b - c
a+b-c+d-e
a + b - c + d - e"""

text = re.sub('(\s?[+-]\s?)', r' \1 ', text)
print(text)

expected result:

a + b
a + b - c
a + b - c
a + b - c + d - e
a + b - c + d - e

<script type="text/javascript" src="//cdn.datacamp.com/dcl-react.js.gz"></script>

<div data-datacamp-exercise data-lang="python">
  <code data-type="sample-code">
    import re

    text = """a+b
      a+b-c
      a + b - c
      a+b-c+d-e
      a + b - c + d - e
      """

    text = re.sub('(\s?[+-]\s?)', r' \1 ', text)
    print(text)
  </code>
</div>

Note that if - or + are consecutive, re.sub('\s?([+-])\s?', r' \1 ', text) won't work well as there will be double spaces in the result. — Wiktor Stribiżew
– Wiktor Stribiżew, Commented Nov 22, 2018 at 9:58

CertainPerformance · Accepted Answer · 2018-11-22 01:46:50Z

5

Try capturing only the [+-], and then replace with that group surrounded by spaces:

text = re.sub('\s?([+-])\s?', r' \1 ', text)

<script type="text/javascript" src="//cdn.datacamp.com/dcl-react.js.gz"></script>

<div data-datacamp-exercise data-lang="python">
  <code data-type="sample-code">
import re

text = """a+b
a+b-c
a + b - c
a+b-c+d-e
a + b - c + d - e
"""

text = re.sub('\s?([+-])\s?', r' \1 ', text)
print(text)
  </code>
</div>

You also might consider repeating the \ss with * instead of ?, so that, for example, 3 + 5 gets prettified to 3 + 5:

\s*([+-])\s*

edited Nov 22, 2018 at 1:46

answered Nov 22, 2018 at 1:41

CertainPerformance

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3 Comments

ewwink Over a year ago

why I'm not thinking about that :lol: will accepted in few minutes, thanks.

ewwink Over a year ago

why \s* can match if there is no space?

CertainPerformance Over a year ago

Because * means repeat the previous token zero or more times (compared to ?, which means repeat the previous token zero or one time)

Ajax1234 · Accepted Answer · 2018-11-22 01:41:11Z

2

You can use a lambda function with re.sub:

import re
new_text = re.sub('(?<=\S)[\+\-](?=\S)', lambda x:f' {x.group()} ', text)

Output:

a + b
a + b - c
a + b - c
a + b - c + d - e
a + b - c + d - e

answered Nov 22, 2018 at 1:41

Ajax1234

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