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Python Advanced Topics – Python RegEx

In this tutorial, you will learn about regular expressions (RegEx), and use Python’s re module to work with RegEx (with the help of examples).

A Regular Expression (RegEx) is a sequence of characters that defines a search pattern. For example,

^a...s$

The above code defines a RegEx pattern. The pattern is: any five letter string starting with a and ending with s.

A pattern defined using RegEx can be used to match against a string.

ExpressionStringMatched?
^a...s$absNo match
aliasMatch
abyssMatch
AliasNo match
An abacusNo match

Python has a module named re to work with RegEx. Here’s an example:

import re

pattern = '^a...s$'
test_string = 'abyss'
result = re.match(pattern, test_string)

if result:
  print("Search successful.")
else:
  print("Search unsuccessful.")	

Here, we used re.match() function to search pattern within the test_string. The method returns a match object if the search is successful. If not, it returns None.


There are other several functions defined in the re module to work with RegEx. Before we explore that, let’s learn about regular expressions themselves.

If you already know the basics of RegEx, jump to Python RegEx.


Specify Pattern Using RegEx

To specify regular expressions, metacharacters are used. In the above example, ^ and $ are metacharacters.


MetaCharacters

Metacharacters are characters that are interpreted in a special way by a RegEx engine. Here’s a list of metacharacters:

[] . ^ $ * + ? {} () \ |


[] – Square brackets

Square brackets specifies a set of characters you wish to match.

ExpressionStringMatched?
[abc]a1 match
ac2 matches
Hey JudeNo match
abc de ca5 matches

Here, [abc] will match if the string you are trying to match contains any of the a, b or c.

You can also specify a range of characters using - inside square brackets.

  • [a-e] is the same as [abcde].
  • [1-4] is the same as [1234].
  • [0-39] is the same as [01239].

You can complement (invert) the character set by using caret ^ symbol at the start of a square-bracket.

  • [^abc] means any character except a or b or c.
  • [^0-9] means any non-digit character.

.Period

A period matches any single character (except newline '\n').

ExpressionStringMatched?
..aNo match
ac1 match
acd1 match
acde2 matches (contains 4 characters)

^Caret

The caret symbol ^ is used to check if a string starts with a certain character.

ExpressionStringMatched?
^aa1 match
abc1 match
bacNo match
^ababc1 match
acbNo match (starts with a but not followed by b)

$Dollar

The dollar symbol $ is used to check if a string ends with a certain character.

ExpressionStringMatched?
a$a1 match
formula1 match
cabNo match

*Star

The star symbol * matches zero or more occurrences of the pattern left to it.

ExpressionStringMatched?
ma*nmn1 match
man1 match
maaan1 match
mainNo match (a is not followed by n)
woman1 match

+Plus

The plus symbol + matches one or more occurrences of the pattern left to it.

ExpressionStringMatched?
ma+nmnNo match (no a character)
man1 match
maaan1 match
mainNo match (a is not followed by n)
woman1 match

?Question Mark

The question mark symbol ? matches zero or one occurrence of the pattern left to it.

ExpressionStringMatched?
ma?nmn1 match
man1 match
maaanNo match (more than one a character)
mainNo match (a is not followed by n)
woman1 match

{}Braces

Consider this code: {n,m}. This means at least n, and at most m repetitions of the pattern left to it.

ExpressionStringMatched?
a{2,3}abc datNo match
abc daat1 match (at daat)
aabc daaat2 matches (at aabc and daaat)
aabc daaaat2 matches (at aabc and daaaat)

Let’s try one more example. This RegEx [0-9]{2, 4} matches at least 2 digits but not more than 4 digits

ExpressionStringMatched?
[0-9]{2,4}ab123csde1 match (match at ab123csde)
12 and 3456733 matches (12, 3456, 73)
1 and 2No match

|Alternation

Vertical bar | is used for alternation (or operator).

ExpressionStringMatched?
a|bcdeNo match
ade1 match (match at ade)
acdbea3 matches (at acdbea)

Here, a|b match any string that contains either a or b


()Group

Parentheses () is used to group sub-patterns. For example, (a|b|c)xz match any string that matches either a or b or c followed by xz

ExpressionStringMatched?
(a|b|c)xzab xzNo match
abxz1 match (match at abxz)
axz cabxz2 matches (at axzbc cabxz)

\Backslash

Backlash \ is used to escape various characters including all metacharacters. For example,

\$a match if a string contains $ followed by a. Here, $ is not interpreted by a RegEx engine in a special way.

If you are unsure if a character has special meaning or not, you can put \ in front of it. This makes sure the character is not treated in a special way.


Special Sequences

Special sequences make commonly used patterns easier to write. Here’s a list of special sequences:

\A – Matches if the specified characters are at the start of a string.

ExpressionStringMatched?
\Athethe sunMatch
In the sunNo match

\b – Matches if the specified characters are at the beginning or end of a word.

ExpressionStringMatched?
\bfoofootballMatch
a footballMatch
afootballNo match
foo\bthe fooMatch
the afoo testMatch
the afootestNo match

\B – Opposite of \b. Matches if the specified characters are not at the beginning or end of a word.

ExpressionStringMatched?
\BfoofootballNo match
a footballNo match
afootballMatch
foo\Bthe fooNo match
the afoo testNo match
the afootestMatch

\d – Matches any decimal digit. Equivalent to [0-9]

ExpressionStringMatched?
\d12abc33 matches (at 12abc3)
PythonNo match

\D – Matches any non-decimal digit. Equivalent to [^0-9]

ExpressionStringMatched?
\D1ab34"503 matches (at 1ab34"50)
1345No match

\s – Matches where a string contains any whitespace character. Equivalent to [ \t\n\r\f\v].

ExpressionStringMatched?
\sPython RegEx1 match
PythonRegExNo match

\S – Matches where a string contains any non-whitespace character. Equivalent to [^ \t\n\r\f\v].

ExpressionStringMatched?
\Sa b2 matches (at a b)
No match

\w – Matches any alphanumeric character (digits and alphabets). Equivalent to [a-zA-Z0-9_]. By the way, underscore _ is also considered an alphanumeric character.

ExpressionStringMatched?
\w12&": ;c3 matches (at 12&": ;c)
%"> !No match

\W – Matches any non-alphanumeric character. Equivalent to [^a-zA-Z0-9_]

ExpressionStringMatched?
\W1a2%c1 match (at 1a2%c)
PythonNo match

\Z – Matches if the specified characters are at the end of a string.

ExpressionStringMatched?
Python\ZI like Python1 match
I like Python ProgrammingNo match
Python is fun.No match

Tip: To build and test regular expressions, you can use RegEx tester tools such as regex101. This tool not only helps you in creating regular expressions, but it also helps you learn it.

Now you understand the basics of RegEx, let’s discuss how to use RegEx in your Python code.


Python RegEx

Python has a module named re to work with regular expressions. To use it, we need to import the module.

import re

The module defines several functions and constants to work with RegEx.


re.findall()

The re.findall() method returns a list of strings containing all matches.


Example 1: re.findall()

# Program to extract numbers from a string

import re

string = 'hello 12 hi 89. Howdy 34'
pattern = '\d+'

result = re.findall(pattern, string) 
print(result)

# Output: ['12', '89', '34']

If the pattern is not found, re.findall() returns an empty list.


re.split()

The re.split method splits the string where there is a match and returns a list of strings where the splits have occurred.


Example 2: re.split()

import re

string = 'Twelve:12 Eighty nine:89.'
pattern = '\d+'

result = re.split(pattern, string) 
print(result)

# Output: ['Twelve:', ' Eighty nine:', '.']

If the pattern is not found, re.split() returns a list containing the original string.


You can pass maxsplit argument to the re.split() method. It’s the maximum number of splits that will occur.

import re

string = 'Twelve:12 Eighty nine:89 Nine:9.'
pattern = '\d+'

# maxsplit = 1
# split only at the first occurrence
result = re.split(pattern, string, 1) 
print(result)

# Output: ['Twelve:', ' Eighty nine:89 Nine:9.']

By the way, the default value of maxsplit is 0; meaning all possible splits.


re.sub()

The syntax of re.sub() is:

re.sub(pattern, replace, string)

The method returns a string where matched occurrences are replaced with the content of replace variable.


Example 3: re.sub()

# Program to remove all whitespaces
import re

# multiline string
string = 'abc 12\
de 23 \n f45 6'

# matches all whitespace characters
pattern = '\s+'

# empty string
replace = ''

new_string = re.sub(pattern, replace, string) 
print(new_string)

# Output: abc12de23f456

If the pattern is not found, re.sub() returns the original string.


You can pass count as a fourth parameter to the re.sub() method. If omited, it results to 0. This will replace all occurrences.

import re

# multiline string
string = 'abc 12\
de 23 \n f45 6'

# matches all whitespace characters
pattern = '\s+'
replace = ''

new_string = re.sub(r'\s+', replace, string, 1) 
print(new_string)

# Output:
# abc12de 23
# f45 6

re.subn()

The re.subn() is similar to re.sub() except it returns a tuple of 2 items containing the new string and the number of substitutions made.


Example 4: re.subn()

# Program to remove all whitespaces
import re

# multiline string
string = 'abc 12\
de 23 \n f45 6'

# matches all whitespace characters
pattern = '\s+'

# empty string
replace = ''

new_string = re.subn(pattern, replace, string) 
print(new_string)

# Output: ('abc12de23f456', 4)

re.search()

The re.search() method takes two arguments: a pattern and a string. The method looks for the first location where the RegEx pattern produces a match with the string.

If the search is successful, re.search() returns a match object; if not, it returns None.

match = re.search(pattern, str)

Example 5: re.search()

import re

string = "Python is fun"

# check if 'Python' is at the beginning
match = re.search('\APython', string)

if match:
  print("pattern found inside the string")
else:
  print("pattern not found")  

# Output: pattern found inside the string

Here, match contains a match object.


Match object

You can get methods and attributes of a match object using dir() function.

Some of the commonly used methods and attributes of match objects are:


match.group()

The group() method returns the part of the string where there is a match.

Example 6: Match object

import re

string = '39801 356, 2102 1111'

# Three digit number followed by space followed by two digit number
pattern = '(\d{3}) (\d{2})'

# match variable contains a Match object.
match = re.search(pattern, string) 

if match:
  print(match.group())
else:
  print("pattern not found")

# Output: 801 35

Here, match variable contains a match object.

Our pattern (\d{3}) (\d{2}) has two subgroups (\d{3}) and (\d{2}). You can get the part of the string of these parenthesized subgroups. Here’s how:

>>> match.group(1)
'801'

>>> match.group(2)
'35'
>>> match.group(1, 2)
('801', '35')

>>> match.groups()
('801', '35')

match.start(), match.end() and match.span()

The start() function returns the index of the start of the matched substring. Similarly, end() returns the end index of the matched substring.

>>> match.start()
2
>>> match.end()
8

The span() function returns a tuple containing start and end index of the matched part.

>>> match.span()
(2, 8)

match.re and match.string

The re attribute of a matched object returns a regular expression object. Similarly, string attribute returns the passed string.

>>> match.re
re.compile('(\\d{3}) (\\d{2})')

>>> match.string
'39801 356, 2102 1111'

We have covered all commonly used methods defined in the re module. If you want to learn more, visit Python 3 re module.


Using r prefix before RegEx

When r or R prefix is used before a regular expression, it means raw string. For example, '\n' is a new line whereas r'\n' means two characters: a backslash \ followed by n.

Backlash \ is used to escape various characters including all metacharacters. However, using r prefix makes \ treat as a normal character.


Example 7: Raw string using r prefix

import re

string = '\n and \r are escape sequences.'

result = re.findall(r'[\n\r]', string) 
print(result)

# Output: ['\n', '\r']

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