Optional characters in a regex
Question
The task is pretty simple, but I've not been able to come up with a good solution yet: a string can contain numbers, dashes and pluses, or only numbers.
^[0-9+-]+$
does most of what I need, except when a user enters garbage like "+-+--+"
I've not had luck with regular lookahead, since the dashes and pluses could potentially be anywhere in the string.
Valid strings:
- 234654
- 24-3+-2
- -234
- 25485+
Invalid:
- ++--+
Solution
How about:
^[0-9+-]*[0-9][0-9+-]*$
This ensures that there is at least one digit somewhere in the string. (It looks like it might have a lot of backtracking, though. But on the other hand it doesn't have a + or * wrapped inside another + or *, which I don't like either.)
OTHER TIPS
How about this:
([+-]?\d[+-]?)+
which means "one or more digits, each of which can be preceded or followed by an optional plus or minus".
Here's a Python test script:
import re
TESTS = "234654 24-3+-2 -234 25485+ ++--+".split()
for test in TESTS:
print test, ":", re.match(r'([+-]?\d[+-]?)+', test) is not None
which prints this:
234654 : True
24-3+-2 : True
-234 : True
25485+ : True
++--+ : False
^([+-]*[0-9]+[+-]*)+$
Another solution using a positive look behind assertion ensuring there is at leat one number.
^[0-9+-]+$(?<=[0-9][+-]*)
Or using a positive look ahead assertion.
(?=[+-]*[0-9])^[0-9+-]+
I like the
^(?=.*\d)[\d+-]+$
solution, myself. It says exactly what you need without requiring any head-scratching.
I'd do it like this:
^[-+]*\d[\d+-]*$
Fast is good!