Regex to replace a list of characters in python

Question

I have a list of characters that I want to find in a string and replace its multiple occurances together into just one occurance.

But I am facing 2 problems - when i loop over them, the re.sub function does not replace the multiple occurances and when i have a smiley like :) it replaces ':' with ':)' which I dont want.

Here is the code that I tried.

end_of_line_chars = [".",";","!",":)",":-)","=)",":]",":-(",":(",":[","=(",":P",":-P",":-p",":p","=P"]
for i in end_of_line_chars:
    pattern = "[" + i + "]" + "+"
    str = re.sub(pattern,i,str)

If I take a single character and try it works as shown below.

str = re.sub("[.]+",".",str)

But looping over a list of characters gives error. How to solve these 2 problems? Thanks for the help.

regex has special characters that you have to escape with a \` in order to actually match those characters. i would suggest using string.replace() method instead, why bother with regex — R Nar
– R Nar, Commented Oct 30, 2015 at 0:06
str.replace won't replace a variable length run of a specific character with a single replacement. — ShadowRanger
– ShadowRanger, Commented Oct 30, 2015 at 0:09

Kijewski · Accepted Answer · 2015-10-30 13:19:00Z

1

re.escape(str) does the escaping for you. Separated with | you can match alternatives. With (?:…) you do grouping without capturing. So:

# Only in Python2:
from itertools import imap as map, ifilter as filter

# Escape all elements for, e.g. ':-)' → r'\:\-\)':
esc = map(re.escape, end_of_line_chars)
# Wrap elements in capturing as group, so you know what element what found,
# and in a non-capturing group with repeats and optional trailing spaces:
esc = map(r'(?:({})\s*)+'.format, esc)
# Compile expressing what finds any of these elements:
esc = re.compile('|'.join(esc))

# The function to turn a match of repeats into a single item:
def replace_with_one(match):
    # match.groups() has captures, where only the found one is truthy: ()
    # e.g. (None, None, None, None, ':-)', None, None, None, None, None, None, None, None, None, None, None)
    return next(filter(bool, match.groups()))

# This is how you use it:
esc.sub(replace_with_one, '.... :-) :-) :-) :-( .....')
# Returns: '.:-):-(.'

edited Oct 30, 2015 at 13:19

answered Oct 30, 2015 at 0:11

Kijewski

26.1k14 gold badges108 silver badges149 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Ram Over a year ago

Hi Kay, Thanks for the answer. But you could tell what do to after getting the esc variable? I am still not able to proceed with that.

ShadowRanger · Accepted Answer · 2015-10-30 00:12:27Z

0

If the things to replace are not single characters, character classes won't work. Instead, use non-capture groups (and use re.escape so the literals aren't interpreted as regex special characters):

end_of_line_chars = [".",";","!",":)",":-)","=)",":]",":-(",":(",":[","=(",":P",":-P",":-p",":p","=P"]
for i in end_of_line_chars:
    pattern = r"(?:{})+".format(re.escape(i))
    str = re.sub(pattern,i,str)

answered Oct 30, 2015 at 0:12

ShadowRanger

158k12 gold badges222 silver badges317 bronze badges

1 Comment

Ram Over a year ago

Hi Shadow Ranger, Thanks for the answer. But your snippet does not give the output I want. Say str contains ":) :) :) :) :) :)" , I want str to change to ":)"

Collectives™ on Stack Overflow

Regex to replace a list of characters in python

2 Answers 2

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related