re.findall在Python中使用分组进行正则表达式失败

Question

Im writing a python program using regex to find email addresses. re.findall function is giving wrong output whenever I try to use round brackets for grouping. Can anyone point out the mistake / suggest an alternate solution?

Here are two snippets of code to explain -

pat = "[\w]+[ ]*@[ ]*[\w]+.[\w]+"
re.findall(pat, 'abc@cs.stansoft.edu.com .rtrt.. myacc@gmail.com ')

gives the output

['abc@cs.stansoft', 'myacc@gmail.com']

However, if I use grouping in this regex and modify the code as

pat = "[\w]+[ ]*@[ ]*[\w]+(.[\w]+)*"
re.findall(pat, 'abc@cs.stansoft.edu.com .rtrt.. myacc@gmail.com ')

the output is

['.com', '.com']

To confirm the correctness of the regex, I tried this specific regex (in second example) in http://regexpal.com/ with the same input string, and both the email addresses are matched successfully.

Answer 1

In Python, re.findall returns the whole match only if there are no groups, if there are groups then it will return the groups. To get around this, you should use a non-capturing group (?:...) . In this case:

pat = "[\w.]+ *@ *\w+(?:\.\w+)*"
re.findall(pat, 'abc@cs.stansoft.edu.com .rtrt.. myacc@gmail.com ')

Answer 2

You would use groups if you wanted to do something like separate the user from the host:
(The hyphens are optional, some emails have them.)

pat = '([\w\.-]+)@([\w\.-]+)'
re.findall(pat, 'abc@cs.stansoft.edu.com .rtrt.. myacc@gmail.com ')

Output:

[('abc', 'cs.stansoft.edu.com'), ('myacc', 'gmail.com')]

To further illustrate we could replace the host, and keep the user from group 1 (\\1):

emails = 'abc@cs.stansoft.edu.com .rtrt.. myacc@gmail.com '
pat = '([\w\.-]+)@([\w\.-]+)'
re.sub(pat, r'\1@live.com', emails)

Output:

'abc@live.com .rtrt.. myacc@live.com '

Simply remove the parentheses from the pattern to match the whole email:

pat = '[\w\.-]+@[\w\.-]+'
re.findall(pat, 'abc@cs.stansoft.edu.com .rtrt.. myacc@gmail.com ')

Output:

['abc@cs.stansoft.edu.com', 'myacc@gmail.com']