CODEWITHSUNDEEP
pythonregex
user3154928
user3154928

Reputation: 23

Why is the '1{0}' regex always matching with an empty string?

import re

if re.match(r'1{0}', 'foo').group() == '':
   print(True)

Could someone explain why the condition is always satisfied?

The first character can be anything like: 1{0}, 2{0}, a{0} etc

And why:

re.match(r'11{0}', 'foo').group()
# AttributeError: 'NoneType' object has no attribute 'group

Upvotes: 2

Views: 78

Answers (2)

NPE
NPE

Reputation: 500713

From the documentation:

{m}

Specifies that exactly m copies of the previous RE should be matched; fewer matches cause the entire RE not to match. For example, a{6} will match exactly six 'a' characters, but not five.

Therefore, 1{0} would match exactly zero repetitions of the character 1. This is the same as an empty regex, and would match anything.

Upvotes: 4

Tim Peters
Tim Peters

Reputation: 70715

Because {0} tells it to match 0 repetitions of what it follows. 0 repetitions is an empty string. An empty string always matches.

Upvotes: 3

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