Regex is a important tool when dealing text. In python we have a library named re.
Most of the regular expression operations are available as module-level function and RegexObject methods.

import the lib

import re

re.compile(pattern, flag=0)

Compile a regular expression pattern into a regular expression object, which can be used for matching using its match() and search() methods, described below.

import re
text = 'I love China'
regexs = [re.compile(p)
        for p in ['love', 'll']
    ]
for regex in regexs:
    print regex.pattern
    if regex.search(text):
        print "Match"
    else:
        print "Not Match"

re.search(pattern, string)

Scan through string looking for a location where the regular expression pattern produces a match, and return a corresponding MatchObject instance.Return None if no position in the string matches the pattern; note that this is different from finding a zero-length match at some point in the string.

#!/usr/bin/python
#encoding:utf-8
import re
text = 'I love China'
regexs = [re.compile(p)
        for p in ['love', 'll']
     ]
for regex in regexs:
    print regex.pattern
    match = re.search(regex, text):
    if match:
        print "Match"
    else:
        print "Not Match"

RegexObject.search(string)

#!/usr/bin/python
#encoding:utf-8
import re
text = 'I love China'
pattern = 'love'
match = re.search(pattern, text)
s = match.start()
e = match.end()
print 'Found "%s"\nin "%s"\nfrom %d to %d ("%s")' % \
      (match.re.pattern, match.string, s, e, text[s:e])

re.match(pattern, srting)

If zero or more characters at the beginning of string match this regular expression, return a corresponding MatchObject instance. Return None if the string does not match the pattern; note that this is different from a zero-length match.

#!/usr/bin/python
#encoding:utf-8
import re
text = 'His phone 12345 number is 67890'
regex = re.compile(r'.*?(\d+).*?(\d+)')
match = re.match(regex, text)
if match: 
    print match.group(1),match.group(2),

RegexObject.match(string)

#!/usr/bin/python
#encoding:utf-8
import re
text = 'His phone 12345 number is 67890'
regex = re.compile(r'.*?(\d+).*?(\d+)')
match = regex.match(text)
if match: 
    print match.group(1), match.group(2),

Match object has many attributes:

Match more than one

re.findall(pattern, string)

#!/usr/bin/python
#encoding:utf-8
import re
text = 'this is a text'
pattern = 'is'
matchs = re.findall(pattern, text)
for match in matchs:
    print match

lo@ubuntu:~/try/regex$ python searchall.py 
is
is

re.finditer(pattern, string)

#!/usr/bin/python
#encoding:utf-8
import re
text = 'this is a text'
pattern = 'is'
matchs = re.finditer(pattern, text)
for match in matchs:
    s = match.start()
    e = match.end()
    print "Found ", text[s:e], "at: ", s, e

lo@ubuntu:~/try/regex$ python searchall.py 
Found  is at:  2 4
Found  is at:  5 7

Pattern Syntax
Repetition {a, b}

* equivalent to {0,}
+ equivalent to {1,}
? equivalent to {0,1}
#!/usr/bin/python
#encoding: utf-8
import re
text = '101000111'
patterns = [
        '10',
        '10?',
        '10*',
        '10+',
        '10{3}',
        '10{1,3}'
        ]

print 'orginal string: ', text

for pattern in patterns:
    matchs = re.finditer(pattern, text)
    for match in matchs:
        s = match.start()
        e = match.end()
        substr = text[s:e]
        print 'pattern: ', pattern,' Found: ', substr, 'at', s, e


@ubuntu:~/try/regex$ python repetition.py 
orginal string:  101000111
pattern:  10  Found:  10 at 0 2
pattern:  10  Found:  10 at 2 4
pattern:  10?  Found:  10 at 0 2
pattern:  10?  Found:  10 at 2 4
pattern:  10?  Found:  1 at 6 7
pattern:  10?  Found:  1 at 7 8
pattern:  10?  Found:  1 at 8 9
pattern:  10*  Found:  10 at 0 2
pattern:  10*  Found:  1000 at 2 6
pattern:  10*  Found:  1 at 6 7
pattern:  10*  Found:  1 at 7 8
pattern:  10*  Found:  1 at 8 9
pattern:  10+  Found:  10 at 0 2
pattern:  10+  Found:  1000 at 2 6
pattern:  10{3}  Found:  1000 at 2 6
pattern:  10{1,3}  Found:  10 at 0 2
pattern:  10{1,3}  Found:  1000 at 2 6

Character set

[a|b] [a-z] [0-9] [a-zA-Z]

#!/usr/bin/python
#encoding utf-8
import re
text = 'string is not 12324, 234 IS NOT STRING'
patterns =[
        '[a-z]+',
        '[A-Z]+',
        '[0-9]+',
        '[a-zA-Z]+',
        '[a-zA-Z0-9]+'
    ]
print "orginal string: ", text

for pattern in patterns:
    print "pattern is: ", pattern
    matchs = re.findall(pattern, text)
    for match in matchs:
        print match


lo@ubuntu:~/try/regex$ python searchset.py 
orginal string:  string is not 12324, 234 IS NOT STRING
pattern is:  [a-z]+
string
is
not
pattern is:  [A-Z]+
IS
NOT
STRING
pattern is:  [0-9]+
12324
234
pattern is:  [a-zA-Z]+
string
is
not
IS
NOT
STRING
pattern is:  [a-zA-Z0-9]+
string
is
not
12324
234
IS
NOT
STRING

Greedy Or Non-greedy(minimal fashion)

Escape sequences #TODO

\d any decimal digit
\D any character that is not a decimal digit
\w any 'word' character
\W any 'non-word' character
\s any whitespace character
\S any character that is not a whitespace character

Anchors

^ the current match point is at the start of the subject string
$ the current match point is at the end of the subject string
\b word boundary
\B not a word boundary
\A start of subject (independent of multiline mode)
\Z end of subject or newline at end (independent of multiline mode)

Reference:

python.org
regex php