Regular expression to check string is integer

Question

An HTML form returns me a string of a number entered by a user. How do I use regular expressions to see if it is capable of being a number or not. I do not simply want to strip away commas and see if it can be cast to int, nor do I like the locale.atoi method as the strings will evalaute to numbers even if they are nonsense (e.g. locale.atoi('01,0,0') evaluates to 100).

NB this validation only occurs if the string contains commas

The re pattern should be:

1st character is 1-9 (not zero) 2nd and 3rd characters are 0-9 Then 3 digits 1-9 and a comma repeated between 0 and 2 times (999,999,999,999 is largest number possible in the program) Then finally 3 digits 1-9

compiled = re.compile("[1-9][0-9]{0,2},(\d\d\d,){0,2}[0-9]{3}")

which is not matching the end of the string correctly, for example:

re.match(compiled, '123,456,78') 

is matching. What have I done wrong?

Answer 1

More Compact

I would suggest something more compact:

^[1-9][0-9]{0,2}(?:,[0-9]{3}){0,3}$

See the demo

• The ^ asserts that we are at the beginning of the string
• [1-9] matches our first digit
• [0-9]{0,2} matches up to two additional digits
• (?:,[0-9]{3}) matches a comma and three digits...
• between 0 and three times
• $ asserts that we are at the end of the string

To validate, you could do:

if re.search("^[1-9][0-9]{0,2}(?:,[0-9]{3}){0,3}$", subject):
    # Successful match
else:
    # Match attempt failed