一个非常简单的正则表达式的问题

Question

I am using python to try to write some simple code that looks through strings with regular expressions and finds things. In this string:

and the next nothing is 44827

I want my regex to return just the numbers.

I have set up my python program like this:

buf = "and the next nothing is 44827"
number = re.search("[0-9]*", buf)
print buf
print number.group()

What number.group() returns is an empty string. However, when the regex is:

number = re.search("[0-9]+", buf)

The full number (44827) is properly extracted. What am I missing here?

Answer 1

The problem is that [0-9]* matches zero or more digits, so it is more than happy to match to a zero-length string.

Meanwhile, [0-9]+ matches one or more digits, so it needs to see at least one number in order to catch.

---

you might want to use findall and handle the case in which you have multiple numbers per line.

Answer 2

Your first regex matches the empty string before the letter "a", so it stops there. Your second doesn't, so it keeps trying.

Answer 3

It's because the first attempt matches an empty string - you're asking it for "0 or more digits" - so the first match is empty at the beginning of the string. When you ask for "one or more digits", the first match starts at the first '4', and continues from there until the end of the number.

Answer 4

See for yourself.

• [0-9]* http://regexr.com?30je4
• [0-9]+ http://regexr.com?30je7

Hint:

• * matches 0-or-more times
• + matches 1-or-more times

Obviously, the first case has more precedence over the second. And the regex engine has NO problem at all, to not match anything. :-)