Regular expression in Python

• re is the Python module to handle regular expressions
• In this tutorial we adapt the following convention:
  • pattern refers to a regular expression pattern
  • string refers to group of characters
  • flags refers to control flags that re module uses to customize its processing

search

• re.search(pattern, string, flag)
  • The search method looks for the first match of the pattern in the string and return a group object.
  • The group object has a span field that contains the beginning and the end indices of the found substring. And its match field contains the found substring

import re # import re module to handle regular expressions 

pattern = "Ali"
string = "My name is Ali"
print( re.search(pattern, string))

<_sre.SRE_Match object; span=(11, 14), match='Ali'>

match

• re.match(pattern, string, flag)
  • The match method is similar to the search method except that it matches the pattern only to the beginning of the string.

pattern = "Ali"
string = "My name is Ali"
print( re.match(pattern, string))

string = "Ali is my name"
print( re.match(pattern, string))

None
<_sre.SRE_Match object; span=(0, 3), match='Ali'>

The group object

• search and match methods return an group object that contains the found substring or None. Therefore, we can test the returned value before processing it.

pattern = "Ali"
string = "My name is Noor"
groupObj = re.search(pattern, string)
print(bool(groupObj))

pattern = "Noor"
groupObj = re.search(pattern, string)
print(bool(groupObj))

False
True

• To access the found substring in the return group object invoke the group() method on the returned object.

pattern = "Noor"
string = "My name is Noor"

groupObj = re.search(pattern, string)
if groupObj:
    print(groupObj.group())

Noor

start/end

• To access the starting (ending) index of the substring in the group object invoke start() (end()) method.

pattern = "Noor"
string = "My name is Noor"

groupObj = re.search(pattern, string)
if groupObj:
    print(groupObj.start())

11

findall

• re.findall(patter, string, flag)
  • findall returns a list of strings that match the pattern in the string

pattern = 'Mohammed|Ali' # regular expression to look for the word Mohammed or Ali
string = "Mohammed is my name. Mohammed Ali is my full name"
print( re.findall(pattern, string))

['Mohammed', 'Mohammed', 'Ali']

Literal Matching

• search, match, and findall search on a substring level
• The search is letter case sensetive, unless the flag re.I is set.

print(re.search('a', 'ali').group())

groupObj =  re.search('a', 'Ali');
if groupObj:
    print('`a` is in `Ali`')
else:
    print('`a` is NOT in `Ali`')
        
groupObj =  re.search('a', 'Ali', re.I)
if groupObj:
    print('`re.I`  ignores letter case. So `a` is in `Ali`')

a
`a` is NOT in `Ali`
`re.I`  ignores letter case. So `a` is in `Ali`

p = re.compile("x*")
p.sub('XY', 'xxx xx x')

'XY XY XY'