Regex in Python: “\d+” and "[0-9]+ do NOT only recognize numbers&Digits but also Operators?

Question

in all referencebooks and forum discussions covering this point I always read that the following expressions "[0-9]" and "\\d" only recognize single digits (and nothing else)

Therefore I would conclude that the regular expression "\\d+" would only recognize strings made up exclusively of digits and nothing else (eg "343535" ).

However, this expression also recognizes strings that could represent mathematical operations.

For example: checking the strings "3-5" or "5+6" for the regex "\\d+" will always return True

How can that be? And how do I have to modify my regex if I want to make sure that strings really ONLY contain digits (and no Operators)?

Answer 1

You need to anchor your regular expressions:

r'^\d+$'

otherwise you'll match part of the string; any string with 1 or more digits will match unless you tell the parser to also look for the start and end of the input string.

Demo:

>>> import re
>>> a_number = re.compile(r'\d+')
>>> only_a_number = re.compile(r'^\d+$')
>>> a_number.search('This string is not a number, but does *contain* 1') is not None
True
>>> only_a_number.search('This string is not a number, but does *contain* 1') is not None
False
>>> a_number.search('1') is not None
True
>>> only_a_number.search('1') is not None
True

Note that I am using re.search() here; if you use re.match() instead an implicit \\A is added to the start of the pattern (match at the beginning of the input string).