为什么'[0-9] *'在我的Python正则表达式中不匹配'abc'，因为字符串中有零个或多个数字？

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 searched for 0 or more digits. It found 0 or more digits. The exact number of digits that it found was 0. Hence, the empty string.

Answer 2

Use a Negated Character Class

In your comment above, you say you expect [0-9]* to match abc because:

"abc" contains 0 digits.

You're misunderstanding what a character class is, and that it contains atoms. Yours is not currently a negative assertion.

You could get a match with [^0-9]* if you don't precompile it. For example:

>>> import re
>>> re.search("[^0-9]*", "abc").group()
'abc'

This would perhaps fit your mental map, but thinking of negated character classes as "not containing a range" as opposed to "not containing any of the included characters" is probably going to lead you astray in the future. YMMV.

Answer 3

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!

Answer 4

becouse your regex looking only for digits and abc doesn't have any digits in it.

in short your regex matches anything with digits and the empty string.

Answer 5

From the documentation , search() does the following:

Scan through string looking for a location where this regular expression produces a match, and return a corresponding match object. Return None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string.

Thus, the fact that m is not None indicates that it found a match. The fact that m.group() returns '' shows what it matched .