How to check if re.sub() has successfully replaced in python? [duplicate]

Answer 1

If you have the following code:

import re
s1 = "aaa"
result = re.sub("a", "b", s1)

You can check if the call to sub made subsitutions by comparing the id of result to s1 like so:

id(s1) == id(result)

or, which is the same:

s1 is result

This is because strings in python are immutable, so if any substitutions are made, the result will be a different string than the original (ie: the original string is unchanged). The advantage of using the ids for comparison rather than the contents of the strings is that the comparison is constant time instead of linear.

Answer 2

You can use re.subn which perform the same operation as sub(), but return a tuple (new_string, number_of_subs_made)

If number of modification is 0 i.e. string is not modified.

>>> re.subn('(xx)+', '', 'abcdab')
('abcdab', 0)
>>> re.subn('(ab)+', '', 'abcdab')
('cd', 2)
>>>