Regex for inserting space after different characters

user_11981271 0
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Would like to add a space in between different characters. The one I have now adds it in-between every character, whereas I would like to omit that if the characters are the same (consecutive).

import re
string = "CEEEETTEEEEGGGCCBCTTBHHHHHCCEEEEEEEEETTEETT"
space = re.compile(r"(?<!^)(?=[CEHBGITS])(?!$)")
print(space.sub("\1", string))

Expected result is: C EEEE TT EEEE GGG CC B C TT B HHHHH CC EEEEEEEEE TT EE TT

python regex string
3 Answers

You can use

re.sub(r'(?<=(.))(?!\1)(?=.)', ' ', str)

Python demo <-\(ツ)/-> Regex demo

The regular expression has the following elements.

(?<=(.)) # positive lookbehind asserts that the current string location
         # is preceded by a character that is saved to capture group 1
(?!\1)   # negative lookahead asserts that the current string
         # location is not followed by the content of capture group 1
(?=.)    # positive lookahead asserts that current string location
         # is followed by a character

The current string location can be thought of as a position between consecutive characters.

user_2569700

Another option is to match the consecutive character using a capture group and a backreference, and in the replacement use the full match denoted by \g<0> predeced by a space.

To prevent a leading space, you can assert that there is at least a preceding character.

(?<=[CEHBGITS])([CEHBGITS])\1*

In parts, the pattern matches:

  • (?<= Positive lookbehind, assert what is directly of left the current position is
    • [CEHBGITS] Match 1 of the lister characters
  • ) Close lookbehind
  • ([CEHBGITS]) Capture group 1, match 1 of the listed characters
  • \1* Optionally repeat what is captured in group 1 using a backreference

See a regex demo and a Python demo.

For example

import re
s = "CEEEETTEEEEGGGCCBCTTBHHHHHCCEEEEEEEEETTEETT"
pattern = r"(?<=[CEHBGITS])([CEHBGITS])\1*"
print(re.sub(pattern, " \g<0>", s))

Output

C EEEE TT EEEE GGG CC B C TT B HHHHH CC EEEEEEEEE TT EE TT

A shorter version matching non whitespace characters:

(?<=\S)(\S)\1*

Regex demo

user_54249880

You can use

re.sub(r'(.)(?=(?!\1).)', r'\1 ', text, flags=re.S)

See the regex demo. Details:

  • (.) - Capturing group 1 (its value is referred to with \1 backreference from the replacement pattern): any one char
  • (?=(?!\1).) - a positive lookahead that requires any single char (.) that is not the same as the char inside Group 1 ((?!\1)) immediately to the right of the current location.

See the Python demo:

import re
text = "CEEEETTEEEEGGGCCBCTTBHHHHHCCEEEEEEEEETTEETT"
print( re.sub(r'(.)(?=(?!\1).)', r'\1 ', text, flags=re.S) )
# => C EEEE TT EEEE GGG CC B C TT B HHHHH CC EEEEEEEEE TT EE TT
user_38329700
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