Python正则表达式-正向前进

Question

str='filename=1817616353&realname=Arguments%20for%20&%20against%20protection%20.pdf&code2=pds'
ptn='(?<=realname=).+(?=&)'
re.search(ptn,str).group()

well, when i run this code i'm expecting to get

'Arguments%20for%20'

as the match, but instead it gives me

'Arguments%20for%20&%20against%20protection%20.pdf'

i thought the match should occur at the first occurrence of '&' , which is right after 'for%20' part, so i have no idea why it's going all the way down to 'pdf' . what am i doing wrong?

Answer 1

Your assumption that the first occurrence of & would match is fundamentally wrong.

.+ means match as many as possible of any character (except newline). Thus this causes anything after it to be matched at the last possible position.

A common fix for "I want as few as possible" is to use a greedy quantifier .+? which means match as few as possible but it could still end up matching things you don't want.

If you really mean "match the first possible & " then the expression you should repeat before it is "anything except & ".

ptn=r'(?<=realname=)[^&]+(?=&)'

(Notice also the use of an r'...' string. It doesn't make any difference here, but it's another common newbie error -- you want backslashes in your regex and don't understand why Python is losing them.)

This is basically a restatement of the other answer on this page but hopefully easier for a beginner to digest.

Answer 2

Use a negated character class instead of .+ :

In [5]: ptn='(?<=realname=)[^&]+(?=&)'

In [6]: re.search(ptn,str).group()
Out[6]: 'Arguments%20for%20'

Although you can use a non greedy quantifier by adding ? at the trailing of .* , but using a negated character class will give you a better performance in this case:

In [7]: ptn='(?<=realname=).+?(?=&)'

In [9]: %timeit re.search(ptn,str).group()
1000000 loops, best of 3: 1.46 us per loop

In [10]: ptn='(?<=realname=)[^&]+(?=&)'

In [11]: %timeit re.search(ptn,str).group()
1000000 loops, best of 3: 1.18 us per loop

For more info read the following post regard the difference between non-greedy quantifier and negated character classes. Which would be better non-greedy regex or negated character class?