Skip to content

Latest commit

 

History

History
141 lines (120 loc) · 3.7 KB

RE.md

File metadata and controls

141 lines (120 loc) · 3.7 KB
Raw

Regular Expresion

What is it?

A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern.

There are various characters, which would have special meaning when they are used in regular expression. To avoid any confusion while dealing with regular expressions, we would use Raw Strings as r'expression'.

Match Function

This function attempts to match RE pattern to string with optional flags.
Here is the syntax for this function −

re.match(pattern, string, flags=0)
Here is the description of the parameters:
Parameter Description
pattern This is the regular expression to be matched.
string This is the string, which would be searched to match the pattern at the beginning of string.
flags You can specify different flags using bitwise OR. These are modifiers, which are listed in the table below.

Example

#!/usr/bin/python
import re

line = "Cats are smarter than dogs"

matchObj = re.match( r'(.*) are (.*?) .*', line, re.M|re.I)

if matchObj:
   print "matchObj.group() : ", matchObj.group()
   print "matchObj.group(1) : ", matchObj.group(1)
   print "matchObj.group(2) : ", matchObj.group(2)
else:
   print "No match!!"

The result obtained will be:-

matchObj.group() :  Cats are smarter than dogs
matchObj.group(1) :  Cats
matchObj.group(2) :  smarter

Match Function

This function searches for first occurrence of RE pattern within string with optional flags.
Here is the syntax for this function:

re.search(pattern, string, flags=0)

The re.search function returns a match object on success, none on failure. We use group(num) or groups() function of match object to get matched expression.

Match Object Methods Description
group(num=0) This method returns entire match (or specific subgroup num)
groups() This method returns all matching subgroups in a tuple (empty if there weren't any)

Example

#!/usr/bin/python
import re

line = "Cats are smarter than dogs";

searchObj = re.search( r'(.*) are (.*?) .*', line, re.M|re.I)

if searchObj:
   print "searchObj.group() : ", searchObj.group()
   print "searchObj.group(1) : ", searchObj.group(1)
   print "searchObj.group(2) : ", searchObj.group(2)
else:
   print "Nothing found!!"

Output:

searchObj.group() :  Cats are smarter than dogs
searchObj.group(1) :  Cats
searchObj.group(2) :  smarter

Match vs Search

match checks for a match only at the beginning of the string, while search checks for a match anywhere in the string.

Example

#!/usr/bin/python
import re

line = "Cats are smarter than dogs";

matchObj = re.match( r'dogs', line, re.M|re.I)
if matchObj:
   print "match --> matchObj.group() : ", matchObj.group()
else:
   print "No match!!"

searchObj = re.search( r'dogs', line, re.M|re.I)
if searchObj:
   print "search --> searchObj.group() : ", searchObj.group()
else:
   print "Nothing found!!"

Output:

No match!!
search --> matchObj.group() :  dogs

Search and Replace

re.sub(pattern, repl, string, max=0)
This method replaces all occurrences of the RE pattern in string with repl, substituting all occurrences unless max provided. This method returns modified string.

Example

#!/usr/bin/python
import re

phone = "2004-959-559 # This is Phone Number"

# Delete Python-style comments
num = re.sub(r'#.*$', "", phone)
print "Phone Num : ", num

# Remove anything other than digits
num = re.sub(r'\D', "", phone)    
print "Phone Num : ", num

Output:

Phone Num :  2004-959-559
Phone Num :  2004959559