Python regular expression question - sub string but not prepended with

:)

I'm trying to sub foo to bar, but开发者_运维问答 only if it's not prepended with ie. /. So...

foobar should change to barbar, but /foobar not.

I've tried to add [^/] at beginning of my re, but that doesn't work if foo is at beginning of string.

I hate regular expressions! :P

Use a negative lookbehind assertion.

>>> re.search('(?<!/)foo', 'foo')
<_sre.SRE_Match object at 0x7f44891518b8>
>>> re.search('(?<!/)foo', '/foo')
>>> re.search('(?<!/)foo', 'barfoo')
<_sre.SRE_Match object at 0x7f4489151850>

try using \bfoo\b

\b is a word boundary, it deals with a lot of common cases like beginning of line, whitespace, etc.