Regex [0-9]* matches empty string on “(2434525)”

I was trying to match all numbers in a string, this is an example code:

re.search(r'[0-9]*', '(123)').group() # outputs: ''

re.search(r'[0-9]*', '123)').group() # outputs: '123'

re.search(r'[0-9]+', '(123)').group() # outputs: '123'

Why is this strange behavior happening here?

Answer

In your case re.search will stop on first match.

re.search(r'[0-9]*', '(123)').group() – will search for digits starting from empty string to infinity long string, and since first char is (, it marches empty string.

re.search(r'[0-9]*', '123)').group() – since first char is digit, it will try to get longest match.

re.search(r'[0-9]+', '(123)').group() – will search for digits starting from at least one char string to infinity long string, and since first char is (, it will skip ( and will start from first digit and will try to get longest match.

* is {0,} and + is {1,}

this is great resource to play with regex https://regex101.com/