Question

🙂

I’m trying to sub foo to bar, but 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! 😛

Answer 1

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>