Question

I’m trying to retrieve the list of indices of each sub-string within a string. This string contains the special character \ several times in different places within the string. The \ should be recognized as a character and not as a special character. When I obtain the starting index of the sub-string it skips over the \ and returns one index less than what it should be. Any help on how to do this would be appreciated.

text = "ab\fx*abcdfansab\fasdafdab\f664s"
for m in re.finditer( 'ab\f', text ):
print( 'll found', m.start(), m.end() )

(‘ll found’, 0, 3)
(‘ll found’, 13, 16)
(‘ll found’, 22, 25)

The second index should be (14, 17) and the third (24, 27). Also, I’m not sure why the first one is right.

Answer 1

Python interpreting the \ as an escape character, like many other programming languages do. If you want a literal backslash, use raw strings, and also double the \ in the pattern, since backslash is a regex metacharacter:

>>> text = r'ab\fx*abcdfansab\fasdafdab\f664s'
>>> for m in re.finditer( r'ab\\f', text ):
...    print( 'll found', m.start(), m.end() )
...
('ll found', 0, 4)
('ll found', 14, 18)
('ll found', 24, 28) 

Alternately, double the backslashes everywhere, and don’t use raw strings. Again, remember to doubly escape in the regex.

>>> text = 'ab\\fx*abcdfansab\\fasdafdab\\f664s'
>>> for m in re.finditer( 'ab\\\\f', text ):
...     print( 'll found', m.start(), m.end() )
... 
('ll found', 0, 4)
('ll found', 14, 18)
('ll found', 24, 28)