Replacing "\"s using re.sub

Andrew M. Kuchling akuchlin at
Sat Feb 19 19:14:04 CET 2000

"Edward C. Jones" <edcjones at> writes:
> I need to replace all instances of "\" in a string with "\\". The
> function "re.sub" looks like the way to do it. "re.sub" is called
> by "re.sub(pattern, repl, string)". For my problem what should
> "pattern" and "repl" be?

If you're only dealing with a substitution of one exact string with
another exact string, it's faster and easier to use string.replace: 

S2 = string.replace(S, '\\', '\\\\')

If you must use regular expressions for some reason (sigh), think
about the problem a bit.  "pattern" needs to match a single \ ; \ is
used as a special character in regular expressions, so to match a
single \ the pattern has to be \\ .

You want the replacement to be \\, but \ is also special in
replacement strings for things like \g<2>, so you need to double them

The patterns need to be inside strings, and Python also uses \ as a
special character; use a raw string, as in r"contents", to prevent
Python from paying attention to \..

S = "String with \\ in it; here's another \\ "
S2 = re.sub(r'\\', r'\\\\', S)
print S
print S2

This outputs:

String with \ in it; here's another \
String with \\ in it; here's another \\

If you didn't use a raw string, you'd need yet another level of \ quoting,
doubling all the backslashes again:

S2 = re.sub('\\\\', '\\\\\\\\', S)

A.M. Kuchling
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