How to replace the first occurrence of a regular expression in Python?

re.sub() has a count parameter that indicates how many substitutions to perform. You can just set that to 1:

>>> s = "foo foo foofoo foo"
>>> re.sub("foo", "bar", s, 1)
'bar foo foofoo foo'
>>> s = "baz baz foo baz foo baz"
>>> re.sub("foo", "bar", s, 1)
'baz baz bar baz foo baz'

Edit: And a version with a compiled SRE object:

>>> s = "baz baz foo baz foo baz"
>>> r = re.compile("foo")
>>> r.sub("bar", s, 1)
'baz baz bar baz foo baz'

Specify the count argument in re.sub(pattern, repl, string[, count, flags])

The optional argument count is the maximum number of pattern occurrences to be replaced; count must be a non-negative integer. If omitted or zero, all occurrences will be replaced.