Regular expression - starting and ending with a letter, accepting only letters, numbers and _

Question

I\'m trying to write a regular expression which specifies that text should start with a letter, every character should be a letter, number or underscore, there should not be

Answer 1

Here's a solution using a negative lookahead (not supported in all regex engines):

^[a-zA-Z](((?!__)[a-zA-Z0-9_])*[a-zA-Z0-9])?$

Test that it works as expected:

import re
tests = [
   ('a', True),
   ('_', False),
   ('zz', True),
   ('a0', True),
   ('A_', False),
   ('a0_b', True),
   ('a__b', False),
   ('a_1_c', True),
]

regex = '^[a-zA-Z](((?!__)[a-zA-Z0-9_])*[a-zA-Z0-9])?$'
for test in tests:
   is_match = re.match(regex, test[0]) is not None
   if is_match != test[1]:
       print "fail: "  + test[0]