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A Guide to Python Regular Expression(Regex)



Hello dear readers! welcome back to another section of our tutorial on Python. In this tutorial post, we are going to be discussing about Python Regular Expressions.

A regular expression is a special set of characters that helps you match or find other strings, using a special syntax held in a pattern. The Regular Expressions are most widely used in UNIX world.

The Python module re provides full support for the perl-like Regex in Python. The re module raises the exception re.error if an error takes place while using or compiling a regular expression.

In this tutorial, we would cover two important functions, that would be made use of in handling regular expressions. But a small thing first: There are various characters, that would have a special meaning when they are used in Regexp. To avoid any confusion while dealing with Regexps, we would use Raw Strings as r'expression'.



The match Function

This function attempts to match RE pattern to string with optional flags.

Syntax

Here is the syntax for this function -

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

Following below is the description of the parameters -

Sr.No.Parameter & Description
1
pattern
This is the regular expression to be matched.
2
string
This is the string, which would be searched to match the pattern at the beginning of string.
3
flags
You can specify different flags using bitwise OR (|). These are modifiers, which are listed in the table below.

Python re.match method returns a match object on success, None on failure. We use the group(num) or groups() function of match object to get matched expression.

Sr.No.Match Object Method & Description
1
group(num=0)
This method returns entire match (or specific subgroup num)
2
groups()
This method returns all matching subgroups in a tuple (empty if there weren't any)

Example

Below is a simple 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!!"

Output

When the above code is executed, it will produce the following result -

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



The Search Function

This function searches for the first occurrence of RE pattern within the string with optional flags.

Syntax

Here is the syntax for this function -

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

Following below is the description of the parameters -
Sr.No.Parameter & Description
1
pattern
This is the regular expression to be matched.
2
string
This is the string, which would be searched to match the pattern anywhere in the string.
3
flags
You can specify different flags using bitwise OR (|). These are modifiers, which are listed in the table below.

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

Sr.No.Match Object Methods & Description
1
group(num=0)
This method returns entire match (or specific subgroup num)
2
groups()
This method returns all matching subgroups in a tuple (empty if there weren't any)

Example

Below is a simple 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

When the above code is executed, it will produce the following result -

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


Matching Versus Searching

Python offers programmers two distinct primitive operations based on Regexp: match checks for a match only at the beginning of the string. While search checks for a match anywhere in the string (this is what Perl programming does by default).

Example

Below is a simple 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

When the above code is executed, it will produce the following result -

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


Search and Replace

One of the most important Python re function that use Regexp is sub.

Syntax

re.sub(pattern, repl, string, max=0)

This Python function replaces all the occurrences of RE pattern in the string with repl, replacing all occurrences unless max provided. This function returns a modified string.

Example

Below is a simple 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

When the above code is executed, it will produce the following result -

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



Regular Expression Modifiers

Python Regexp literals may include an optional modifier to control different aspects of matching. The modifiers are been specified as an optional flag. Multiple modifier can be provided using exclusive OR (|), as shown previously and can be represented by one of these -

Sr.No.Modifier & Description
1
re.I
Performs case-insensitive matching.
2
re.L
Interprets words according to the current locale. This interpretation affects the alphabetic group (\w and \W), as well as word boundary behavior(\b and \B).
3
re.M
Makes $ match the end of a line (not just the end of the string) and makes ^ match the start of any line (not just the start of the string).
4
re.S
Makes a period (dot) match any character, including a newline.
5
re.U
Interprets letters according to the Unicode character set. This flag affects the behavior of \w, \W, \b, \B.
6
re.X
Permits "cuter" regular expression syntax. It ignores whitespace (except inside a set [] or when escaped by a backslash) and treats unescaped # as a comment marker.


Regular Expression Patterns

Except for control characters, (+ ? . * ^ $ ( ) [ ] { } | \), all characters match themselves. A Python control character can be escaped by preceding the control character with a backslash.

Following table list the regular expression syntax that is available in Python -

Sr.No.Pattern & Description
1
^
Matches beginning of line.
2
$
Matches end of line.
3
.
Matches any single character except newline. Using m option allows it to match newline as well.
4
[...]
Matches any single character in brackets.
5
[^...]
Matches any single character not in brackets
6
re*
Matches 0 or more occurrences of preceding expression.
7
re+
Matches 1 or more occurrence of preceding expression.
8
re?
Matches 0 or 1 occurrence of preceding expression.
9
re{ n}
Matches exactly n number of occurrences of preceding expression.
10
re{ n,}
Matches n or more occurrences of preceding expression.
11
re{ n, m}
Matches at least n and at most m occurrences of preceding expression.
12
a| b
Matches either a or b.
13
(re)
Groups regular expressions and remembers matched text.
14
(?imx)
Temporarily toggles on i, m, or x options within a regular expression. If in parentheses, only that area is affected.
15
(?-imx)
Temporarily toggles off i, m, or x options within a regular expression. If in parentheses, only that area is affected.
16
(?: re)
Groups regular expressions without remembering matched text.
17
(?imx: re)
Temporarily toggles on i, m, or x options within parentheses.
18
(?-imx: re)
Temporarily toggles off i, m, or x options within parentheses.
19
(?#...)
Comment.
20
(?= re)
Specifies position using a pattern. Doesn't have a range.
21
(?! re)
Specifies position using pattern negation. Doesn't have a range.
22
(?> re)
Matches independent pattern without backtracking.
23
\w
Matches word characters.
24
\W
Matches nonword characters.
25
\s
Matches whitespace. Equivalent to [\t\n\r\f].
26
\S
Matches nonwhitespace.
27
\d
Matches digits. Equivalent to [0-9].
28
\D
Matches nondigits.
29
\A
Matches beginning of string.
30
\Z
Matches end of string. If a newline exists, it matches just before newline.
31
\z
Matches end of string.
32
\G
Matches point where last match finished.
33
\b
Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets.
34
\B
Matches nonword boundaries.
35
\n, \t, etc.
Matches newlines, carriage returns, tabs, etc.
36
\1...\9
Matches nth grouped subexpression.
37
\10
Matches nth grouped subexpression if it matched already. Otherwise refers to the octal representation of a character code.


Regular Expression Examples

Literal characters

Sr.No.Example & Description
1
python
Match "python".

Character classes

Sr.No.Example & Description
1
[Pp]ython
Match "Python" or "python"
2
rub[ye]
Match "ruby" or "rube"
3
[aeiou]
Match any one lowercase vowel
4
[0-9]
Match any digit; same as [0123456789]
5
[a-z]
Match any lowercase ASCII letter
6
[A-Z]
Match any uppercase ASCII letter
7
[a-zA-Z0-9]
Match any of the above
8
[^aeiou]
Match anything other than a lowercase vowel
9
[^0-9]
Match anything other than a digit

Special Character Classes

Sr.No.Example & Description
1
.
Match any character except newline
2
\d
Match a digit: [0-9]
3
\D
Match a nondigit: [^0-9]
4
\s
Match a whitespace character: [ \t\r\n\f]
5
\S
Match nonwhitespace: [^ \t\r\n\f]
6
\w
Match a single word character: [A-Za-z0-9_]
7
\W
Match a nonword character: [^A-Za-z0-9_]


Repetition Cases

Sr.No.Example & Description
1
ruby?
Match "rub" or "ruby": the y is optional
2
ruby*
Match "rub" plus 0 or more ys
3
ruby+
Match "rub" plus 1 or more ys
4
\d{3}
Match exactly 3 digits
5
\d{3,}
Match 3 or more digits
6
\d{3,5}
Match 3, 4, or 5 digits

Nongreedy repetition

This matches the smallest number of repetition -

Sr.No.Example & Description
1
<.*>
Greedy repetition: matches "<python>perl>"
2
<.*?>
Nongreedy: matches "<python>" in "<python>perl>"

Grouping with Parentheses

Sr.No.Example & Description
1
\D\d+
No group: + repeats \d
2
(\D\d)+
Grouped: + repeats \D\d pair
3
([Pp]ython(, )?)+
Match "Python", "Python, python, python", etc.

Backreferences
This matches a formerly matched group again -

Sr.No.Example & Description
1
([Pp])ython&\1ails
Match python&pails or Python&Pails
2
(['"])[^\1]*\1
Single or double-quoted string. \1 matches whatever the 1st group matched. \2 matches whatever the 2nd group matched, etc.

Alternatives
Sr.No.Example & Description
1
python|perl
Match "python" or "perl"
2
rub(y|le))
Match "ruby" or "ruble"
3
Python(!+|\?)
"Python" followed by one or more ! or one ?

RECOMMENDED: Python Pass Statement

Anchors
This needs to specify the position of match -

Sr.No.Example & Description
1
^Python
Match "Python" at the start of a string or internal line
2
Python$
Match "Python" at the end of a string or line
3
\APython
Match "Python" at the start of a string
4
Python\Z
Match "Python" at the end of a string
5
\bPython\b
Match "Python" at a word boundary
6
\brub\B
\B is nonword boundary: match "rub" in "rube" and "ruby" but not alone
7
Python(?=!)
Match "Python", if followed by an exclamation point.
8
Python(?!!)
Match "Python", if not followed by an exclamation point.

Special Syntax with Parentheses
Sr.No.Example & Description
1
R(?#comment)
Matches "R". All the rest is a comment
2
R(?i)uby
Case-insensitive while matching "uby"
3
R(?i:uby)
Same as above
4
rub(?:y|le))
Group only without creating \1 backreference

RECOMMENDED: Python Numbers

Alright guys! This is where we are rounding up for this tutorial post. In our next tutorial, we are going to be studying about the Python CGI Programing.

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