Python regex for fixed character length

Question

I am new to regex am trying to create a simple regex. I have gotten below so far which works for the format I'm trying to test against.

import re
pattern = '^\+61-' #to ensure that the given string starts with +61
x = re.search(pattern, '+61-457999999')
print(x)

Output:

<re.Match object; span=(0, 4), match='+61-'>

What I'd like to do next is to add the character count check. So I tried to add {} to the end of my pattern. But this doesn't seem to work. I tried various combinations:

E.g. '^\+61-{1}' - seems to look for 1 number of occurrence of '+61-' at the beginning.

What would be an appropriate addition to the regex so:

The starting characters are always '+61-4'
The length of the given input is always 13

This sounds like a simple question but I was unable to find an exact matching answer for Python and the scenario described.

ggorlen · Accepted Answer · 2021-03-29 23:53:50Z

2

A general solution would be to match the length with a lookahead: (?=^.{13}$). Full example:

>>> bool(re.search(r"(?=^.{13}$)^\+61-", '+61-457999999'))
True
>>> bool(re.search(r"(?=^.{13}$)^\+61-", '+62-457999999'))
False
>>> bool(re.search(r"(?=^.{13}$)^\+61-", '+61-4579999999'))
False
>>> bool(re.search(r"(?=^.{13}$)^\+61-", '+61-45799999'))
False

You could also be more precise and match the format, assuming digits after the -:

>>> bool(re.fullmatch(r"\+61-\d{9}", '+61-457999999'))
True
>>> bool(re.fullmatch(r"\+61-\d{9}", '+62-457999999'))
False
>>> bool(re.fullmatch(r"\+61-\d{9}", '+61-4579999999'))
False
>>> bool(re.fullmatch(r"\+61-\d{9}", '+61-45799999'))
False

Or use .{9} if you want to match anything for the remaining 9 characters after the starting substring.

The reason '^\+61-{1}' doesn't work is it's specifying 1 occurrence of the preceding character -. {1} is always implicit after every character so that's no different than '^\+61-'.

As an aside, always use raw strings r"" for regex patterns in Python.

As another aside, you're in Python so it's easy to check the string's length with len.

edited Mar 29, 2021 at 23:53

answered Mar 29, 2021 at 23:39

ggorlen

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1 Comment

anoj-cha Over a year ago

Perfect. Thank you! Great descriptions on the comments. And yes I've been using len separately and was wondering if it would be better to do it all in one regex. Thanks again for the detailed info. Appreciate it.

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