Replace first occurrence of string in Python

Use re.sub directly, this allows you to specify a count:

regex.sub('', url, 1)

(Note that the order of arguments is replacement, original not the opposite, as might be suspected.)

string replace() function perfectly solves this problem:

string.replace(s, old, new[, maxreplace])

Return a copy of string s with all occurrences of substring old replaced by new. If the optional argument maxreplace is given, the first maxreplace occurrences are replaced.

>>> u'longlongTESTstringTEST'.replace('TEST', '?', 1)
u'longlong?stringTEST'