如何通过正则表达式获得最正确的匹配？

Question

I think this is a common problem. But I didn't find a satisfactory answer elsewhere.

Suppose I extract some links from a website. The links are like the following:

http://example.com/goto/http://example1.com/123.html
http://example1.com/456.html
http://example.com/yyy/goto/http://example2.com/789.html
http://example3.com/xxx.html

I want to use regular expression to convert them to their real links:

http://example1.com/123.html
http://example1.com/456.html
http://example2.com/789.html
http://example3.com/xxx.html

However, I can't do that because of the greedy feature of RE. 'http://.*$' will only match the whole sentence. Then I tried 'http://.*?$' but it didn't work either. Nor did re.findall . So is there any other way to do this?

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Yes. I can do it by str.split or str.index . But I'm still curious about whether there is a RE solution for this.

Answer 1

You don't need to use regex you can use str.split() to split your links with // then pickup the last part and concatenate that with http// :

>>> s="""http://example.com/goto/http://example1.com/123.html
... http://example1.com/456.html
... http://example.com/yyy/goto/http://example2.com/789.html
... http://example3.com/xxx.html"""
>>> ['http://'+s.split('//')[-1] for link in s.split('\n')]
['http://example3.com/xxx.html', 'http://example3.com/xxx.html', 'http://example3.com/xxx.html', 'http://example3.com/xxx.html']

And with regex you just need to replace all characters between 2 // with empty string but as you need one of // for the first use a positive look-behind :

>>> [re.sub(r'(?<=//)(.*)//','',link) for link in s.split('\n')]
['http://example1.com/123.html', 'http://example1.com/456.html', 'http://example2.com/789.html', 'http://example3.com/xxx.html']
>>> 

Answer 2

use this pattern

 ^(.*?[^/])(?=\\/[^/]).*?([^/]+)$ 

and replace with $1/$2
Demo

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after reading comment below, use this pattern to capture what you want

(http://(?:[^h]|h(?!ttp:))*)$

Demo

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or this pattern

(http://(?:(?!http:).)*)$  

Demo

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or this pattern

http://.*?(?=http://)  

and replace with nothing
Demo