[python] Python extract pattern matches

Python 2.7.1 I am trying to use python regular expression to extract words inside of a pattern

I have some string that looks like this

someline abc
someother line
name my_user_name is valid
some more lines

I want to extract the word "my_user_name". I do something like

import re
s = #that big string
p = re.compile("name .* is valid", re.flags)
p.match(s) #this gives me <_sre.SRE_Match object at 0x026B6838>

How do I extract my_user_name now?

You can use groups (indicated with '(' and ')') to capture parts of the string. The match object's group() method then gives you the group's contents:

>>> import re
>>> s = 'name my_user_name is valid'
>>> match = re.search('name (.*) is valid', s)
>>> match.group(0)  # the entire match
'name my_user_name is valid'
>>> match.group(1)  # the first parenthesized subgroup
'my_user_name'

In Python 3.6+ you can also index into a match object instead of using group():

>>> match[0]  # the entire match 
'name my_user_name is valid'
>>> match[1]  # the first parenthesized subgroup
'my_user_name'

You can also use a capture group (?P<user>pattern) and access the group like a dictionary match['user'].

string = '''someline abc\n
            someother line\n
            name my_user_name is valid\n
            some more lines\n'''

pattern = r'name (?P<user>.*) is valid'
matches = re.search(pattern, str(string), re.DOTALL)
print(matches['user'])

# my_user_name

You want a capture group.

p = re.compile("name (.*) is valid", re.flags) # parentheses for capture groups
print p.match(s).groups() # This gives you a tuple of your matches.

It seems like you're actually trying to extract a name vice simply find a match. If this is the case, having span indexes for your match is helpful and I'd recommend using re.finditer. As a shortcut, you know the name part of your regex is length 5 and the is valid is length 9, so you can slice the matching text to extract the name.

Note - In your example, it looks like s is string with line breaks, so that's what's assumed below.

## covert s to list of strings separated by line:
s2 = s.splitlines()

## find matches by line: 
for i, j in enumerate(s2):
    matches = re.finditer("name (.*) is valid", j)
    ## ignore lines without a match
    if matches:
        ## loop through match group elements
        for k in matches:
            ## get text
            match_txt = k.group(0)
            ## get line span
            match_span = k.span(0)
            ## extract username
            my_user_name = match_txt[5:-9]
            ## compare with original text
            print(f'Extracted Username: {my_user_name} - found on line {i}')
            print('Match Text:', match_txt)

You can use matching groups:

p = re.compile('name (.*) is valid')

e.g.

>>> import re
>>> p = re.compile('name (.*) is valid')
>>> s = """
... someline abc
... someother line
... name my_user_name is valid
... some more lines"""
>>> p.findall(s)
['my_user_name']

Here I use re.findall rather than re.search to get all instances of my_user_name. Using re.search, you'd need to get the data from the group on the match object:

>>> p.search(s)   #gives a match object or None if no match is found
<_sre.SRE_Match object at 0xf5c60>
>>> p.search(s).group() #entire string that matched
'name my_user_name is valid'
>>> p.search(s).group(1) #first group that match in the string that matched
'my_user_name'

As mentioned in the comments, you might want to make your regex non-greedy:

p = re.compile('name (.*?) is valid')

to only pick up the stuff between 'name ' and the next ' is valid' (rather than allowing your regex to pick up other ' is valid' in your group.

Maybe that's a bit shorter and easier to understand:

import re
text = '... someline abc... someother line... name my_user_name is valid.. some more lines'
>>> re.search('name (.*) is valid', text).group(1)
'my_user_name'

Here's a way to do it without using groups (Python 3.6 or above):

>>> re.search('2\d\d\d[01]\d[0-3]\d', 'report_20191207.xml')[0]
'20191207'

You could use something like this:

import re
s = #that big string
# the parenthesis create a group with what was matched
# and '\w' matches only alphanumeric charactes
p = re.compile("name +(\w+) +is valid", re.flags)
# use search(), so the match doesn't have to happen 
# at the beginning of "big string"
m = p.search(s)
# search() returns a Match object with information about what was matched
if m:
    name = m.group(1)
else:
    raise Exception('name not found')