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

Question

Since re.sub() returns the whole modified/unmodified string, is there any way to check if re.sub() has successfully modified the text, without searching the output of re.sub()?

Answer 1

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)
>>> 

Answer 2

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.