Why doesn't '[0-9]*' match 'abc' in my Python regular expression since there are zero or more digits in the string?

Question

Why does this regex:

>>> r = re.compile("[0-9]*", re.DEBUG)

match like this:

>>> m = r.search("abc")
>>> m.group()
''

I was hoping that it would match the entire string 'abc' since 'a' fulfills the condition viz match 0 digits, and then the greedy match would include the string 'abc' in its entirety.

Answer 1

You asked "find me zero or more digits", so it found you zero or more digits (zero; empty string).

If you wanted "find me zero or more digits followed by zero or more other characters", you need to say that (with the .* pattern). '[0-9]*' does not match 'abc', because 'abc' includes characters (letters) not included in the requested expression.

>>> r = re.compile('[0-9]*.*')  # Note the very important ".*" that matches everything!
>>> r.search('abc').group()
'abc'

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The point is the word "match". If your expression does not contain [a representation of] a certain character (such as "a"), then it cannot possibly match a string that contains that character! Your given expression matches only strings consisting of zero or more digits and nothing else. Therefore it clearly doesn't match 'abc'.

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As Tigerhawk has mentioned in the comments, if the * in regular expressions meant "zero or more of the preceding pattern, or anything else", it would be extraordinarily useless, as any pattern with a * in it would match all strings, all the time!