CodeQL documentation

Missing part of special group in regular expression

ID: py/regex/incomplete-special-group
Kind: problem
Severity: warning
Precision: high
   - reliability
   - correctness
Query suites:
   - python-security-and-quality.qls

Click to see the query in the CodeQL repository

One of the problems with using regular expressions is that almost any sequence of characters is a valid pattern. This means that it is easy to omit a necessary character and still have a valid regular expression. Omitting a character in a named capturing group is a specific case which can dramatically change the meaning of a regular expression.


Examine the regular expression to find and correct any typos.


In the following example, the regular expression for matcher, r"(P<name>[\w]+)", is missing a “?” and will match only strings of letters that start with “P<name>”, instead of matching any sequence of letters and placing the result in a named group. The fixed version, fixed_matcher, includes the “?” and will work as expected.

import re
matcher = re.compile(r'(P<name>[\w]+)')

def only_letters(text):
    m = matcher.match(text)
    if m:
        print("Letters are: " +'name'))

#Fix the pattern by adding the missing '?'
fixed_matcher = re.compile(r'(?P<name>[\w]+)')