Handling backreferences to capturing groups in re.sub replacement pattern

Question

I want to take the string 0.71331, 52.25378 and return 0.71331,52.25378 - i.e. just look for a digit, a comma, a space and a digit, and strip out the space.

This is my current code:

coords = '0.71331, 52.25378' coord_re = re.sub("(\d), (\d)", "\1,\2", coords) print coord_re 

But this gives me 0.7133,2.25378. What am I doing wrong?

Answer 1

You should be using raw strings for regex, try the following:

coord_re = re.sub(r"(\d), (\d)", r"\1,\2", coords) 

With your current code, the backslashes in your replacement string are escaping the digits, so you are replacing all matches the equivalent of chr(1) + "," + chr(2):

>>> '\1,\2' '\x01,\x02' >>> print '\1,\2' , >>> print r'\1,\2'   # this is what you actually want \1,\2 

Any time you want to leave the backslash in the string, use the r prefix, or escape each backslash (\\1,\\2).

Answer 2

Python interprets the \1 as a character with ASCII value 1, and passes that to sub.

Use raw strings, in which Python doesn't interpret the \.

coord_re = re.sub(r"(\d), (\d)", r"\1,\2", coords) 

This is covered right in the beginning of the re documentation, should you need more info.