Python Forum

How do I check if the first X characters of a string are numbers?

For example if someone inputs a combination of numbers and characters, how can I check to see if the first 3 characters are numbers?

You could use string splice and isdigit

Or a regular expression.

https://docs.python.org/3/library/re.html

import re

for string in ('123ab', '12.3ab', '12', '123', 'a123'):
    print(string, re.match(r'\d{3}.*', string))

Output:
123ab <re.Match object; span=(0, 5), match='123ab'>
12.3ab None
12 None
123 <re.Match object; span=(0, 3), match='123'>
a123 None

OK I figured it out.

String splice + check. (menator01's solution)

Simple.

Thanks for the replies.

All the best.

menator's solution doesn't work if the string is shorter than 3 characters.

for string in ("123ab", "12.3ab", "12", "123", "a123"):
    print(string, string[:3].isdigit())

Output:
123ab True
12.3ab False
12 True
123 True
a123 False

12 passes as a string that starts with 3 digits.

Could use an extra condition check

for string in ('123ab', '12.3ab', '12', '123', 'a123'):
    if string[:3].isdigit() and len(string[:3]) >= 3:
        print(string)

output:

Output:
123ab
123

Minor point that hopefully does not apply - neither solution works for negative numbers.