Python RegEx

Python RegEx#

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.

Expression	String	Matched?
	`abs`	No match
	`alias`	Match
`^a...s$`	`abyss`	Match
	`Alias`	No match
	`An abacus`	No 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!")	

Search successful.

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.

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:

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

1. `[]` - Square brackets#

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

Expression	String	Matched?
	`a`	1 match
	`ac`	2 matches
`[abc]`	`Hey Jude`	No match
	`abc de ca`	5 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-c] means, a or b or c
[a-z] means, any letter from a to z
[A-Z] means, any character from A to Z
[0-3] means, 0 or 1 or 2 or 3
[0-9] means any number from 0 to 9
[A-Za-z0-9] any single character, that is a to z, A to Z or 0 to 9

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.

2. `.` - Period#

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

Expression	String	Matched?
	`a`	No match
	`ac`	1 match
`..`	`acd`	1 match
	`acde`	2 matches (contains 4 characters)

3. `^` - Caret#

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

r'^substring' eg r'^love', a sentence that starts with a word love
r'[^abc] means not a, not b, not c.

Expression	String	Matched?
	`a`	1 match
`^a`	`abc`	1 match
	`bac`	No match
———-	————	————————————————————
`^a`	`abc`	1 match
	`acd`	No match (starts with `a` but not followed by `b`)

4. `$` - Dollar#

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

Expression	String	Matched?
	`a`	1 match
`a$`	`formula`	1 match
	`cab`	No result

5. `*` - Star#

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

Expression	String	Matched?
	`mn`	1 match
	`man`	1 match
*`man`**	`maaan`	1 match
	`main`	No match (`a` is not followed by `n`)
	`woman`	1 match

6. `+` - Plus#

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

Expression	String	Matched?
	`mn`	No match (no `a` character)
	`man`	1 match
`ma+n`	`maaan`	1 match
	`main`	No match (`a` is not followed by `n`)
	`woman`	1 match

7. `?` - Question Mark#

The question mark symbol ? matches zero or more occurrences of the pattern left to it.

Expression	String	Matched?
	`mn`	1 match
	`man`	1 match
`ma?n`	`maaan`	No match (more than one `a` character)
	`main`	No match (`a` is not followed by `n`)
	`woman`	1 match

8. `{}` - Braces#

We can specify the length of the substring we are looking for in a text, using a curly brackets {}.

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

{3}: Exactly 3 characters
{3,}: At least 3 characters
{3,8}: 3 to 8 characters

Expression	String	Matched?
	`abc dat`	No match
	`abc daat`	1 match (at `daat`)
`a{2,3}`	`aabc daaat`	2 matches (at `aabc` and `daaat`)
	`aabc daaaat`	2 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

Expression	String	Matched?
	`ab123csde`	1 match (match at `ab123csde`)
`[0-9]{2,4}`	`12 and 345673`	3 matches (`12`, `3456`, `73`)
	`1 and 2`	No match

9. `|` - Alternation#

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

Expression	String	Matched?
	`cde`	No match
`a\|b`	`ade`	1 match (match at `ade`)
	`acdbea`	3 matches (at `acdbea`)

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

10. `()` - 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.

Expression	String	Matched?
	`ab xz`	No match
`(a\|b\|c)xz`	`abxz`	1 match (match at `abxz`)
	`axz cabxz`	2 matches (at `axz cabxz`)

11. `\` - 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.#

Expression	String	Matched?
`\Athe`	`the sun`	Match
	`In the sun`	No match

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

Expression	String	Matched?
	`football`	Match
`\bfoo`	`a football`	Match
	`afootball`	No match
————-	————	———–
`foo\b`	`the foo`	Match
	`the afoo test`	Match
	`the afootest`	No match

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

Expression	String	Matched?
	`football`	No match
`\Bfoo`	`a football`	No match
	`afootball`	Match
————-	————	———–
`foo\B`	`the foo`	No match
	`the afoo test`	No match
	`the afootest`	Match

`\d` - Matches any non-decimal digit. Equivalent to `[^0-9]`#

\d means: match where the string contains digits (numbers from 0-9)
\D means: match where the string does not contain digits

Expression	String	Matched?
`\d`	`1ab34"50`	3 matches (at `1ab34"50`)
	`1345`	No match

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

Expression	String	Matched?
`\s`	`Python RegEx`	1 match
	`PythonRegEx`	No match

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

Expression	String	Matched?
`\S`	`a b`	2 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.#

Expression	String	Matched?
`\w`	`12&": ;c`	3 matches (at `12&": ;c`)
	`%"> !`	No match

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

Expression	String	Matched?
`\W`	`1a2%c`	1 match (at `1a2%c`)
	`Python`	No match

`\Z` - Matches if the specified characters are at the end of a string.#

Expression	String	Matched?
	`I like Python`	1 match
`Python\Z`	`I like Python Programming`	No match
	`Python is fun.`	No match

Summary - MetaCharacters#

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.

A Regular Expression or RegEx is a special text string that helps to find patterns in data. A RegEx can be used to check if some pattern exists in a different data type. To use RegEx in python first we should import the RegEx module which is called re.

Python RegEx Methods#

To find a pattern we use different set of re character sets that allows to search for a match in a string.

re.findall: Returns a list containing all matches
re.split: Takes a string, splits it at the match points, returns a list
re.sub: Replaces one or many matches within a string
re.search: Returns a match object if there is one anywhere in the string, including multiline strings.
re.match(): searches only in the beginning of the first line of the string and returns matched objects if found, else returns None.

To use it, we need to import the module.

import re

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

1. `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']

['12', '89', '34']

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

# Example 2: re.findall()

txt = '''Python is the most beautiful language that a human being has ever created.
I recommend python for a first programming language'''

# It return a list
matches = re.findall('language', txt, re.I)
print(matches)  # ['language', 'language']

['language', 'language']

As you can see, the word language was found two times in the string. Let us practice some more. Now we will look for both Python and python words in the string:

# Example 3: re.findall()

txt = '''Python is the most beautiful language that a human being has ever created.
I recommend python for a first programming language'''

# It returns list
matches = re.findall('python', txt, re.I)
print(matches)  # ['Python', 'python']

['Python', 'python']

Since we are using re.I both lowercase and uppercase letters are included. If we do not have the re.I flag, then we will have to write our pattern differently. Let us check it out:

# Example 4: re.findall()

txt = '''Python is the most beautiful language that a human being has ever created.
I recommend python for a first programming language'''

matches = re.findall('Python|python', txt)
print(matches)  # ['Python', 'python']

matches = re.findall('[Pp]ython', txt)
print(matches)  # ['Python', 'python']

['Python', 'python']
['Python', 'python']

2. `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 1: re.split()

txt = '''I am teacher and  I love teaching.
There is nothing as rewarding as educating and empowering people.
I found teaching more interesting than any other jobs.
Does this motivate you to be a teacher?'''
print(re.split('\n', txt)) # splitting using \n - end of line symbol

['I am teacher and  I love teaching.', 'There is nothing as rewarding as educating and empowering people.', 'I found teaching more interesting than any other jobs.', 'Does this motivate you to be a teacher?']

# 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:', '.']

['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.

# Example 3: re.split()

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.']

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

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

3. `re.sub()`#

Syntax:

re.sub(pattern, replace, string)

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

# Example 1: 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

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.

# Example 2: re.sub()

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

abc12de 23 
 f45 6

# Example 3: re.sub()

txt = '''Python is the most beautiful language that a human being has ever created.
I recommend python for a first programming language'''

match_replaced = re.sub('Python|python', 'JavaScript', txt, re.I)
print(match_replaced)  # JavaScript is the most beautiful language that a human being has ever created.
# OR
match_replaced = re.sub('[Pp]ython', 'JavaScript', txt, re.I)
print(match_replaced)  # JavaScript is the most beautiful language that a human being has ever created.

JavaScript is the most beautiful language that a human being has ever created.
I recommend JavaScript for a first programming language
JavaScript is the most beautiful language that a human being has ever created.
I recommend JavaScript for a first programming language

Let us add one more example. The following string is really hard to read unless we remove the % symbol. Replacing the % with an empty string will clean the text.

# Example 4: re.sub()

txt = '''%I a%m te%%a%%che%r% a%n%d %% I l%o%ve te%ach%ing. 
T%he%re i%s n%o%th%ing as r%ewarding a%s e%duc%at%i%ng a%n%d e%m%p%ow%er%ing p%e%o%ple.
I fo%und te%a%ching m%ore i%n%t%er%%es%ting t%h%an any other %jobs. 
D%o%es thi%s m%ot%iv%a%te %y%o%u to b%e a t%e%a%cher?'''

matches = re.sub('%', '', txt)
print(matches)

I am teacher and  I love teaching. 
There is nothing as rewarding as educating and empowering people.
I found teaching more interesting than any other jobs. 
Does this motivate you to be a teacher?

4. `re.subn()`#

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

# Example 1: 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)

('abc12de23f456', 4)

5. `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 1: 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

pattern found inside the string

Here, match contains a match object.

# Example 2: re.search()

import re

txt = '''Python is the most beautiful language that a human being has ever created.
I recommend python for a first programming language'''

# It returns an object with span and match
match = re.search('first', txt, re.I)
print(match)  # <re.Match object; span=(100, 105), match='first'>
# We can get the starting and ending position of the match as tuple using span
span = match.span()
print(span)     # (100, 105)
# Lets find the start and stop position from the span
start, end = span
print(start, end)  # 100 105
substring = txt[start:end]
print(substring)       # first

<re.Match object; span=(100, 105), match='first'>
(100, 105)
100 105
first

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:

import re

txt = 'I love to teach python and javaScript'
# It returns an object with span, and match
match = re.match('I love to teach', txt, re.I)
print(match)  # <re.Match object; span=(0, 15), match='I love to teach'>
# We can get the starting and ending position of the match as tuple using span
span = match.span()
print(span)     # (0, 15)
# Lets find the start and stop position from the span
start, end = span
print(start, end)  # 0, 15
substring = txt[start:end]
print(substring)       # I love to teach

<re.Match object; span=(0, 15), match='I love to teach'>
(0, 15)
0 15
I love to teach

As you can see from the example above, the pattern we are looking for (or the substring we are looking for) is I love to teach. The match function returns an object only if the text starts with the pattern.

import re

txt = 'I love to teach python and javaScript'
match = re.match('I like to teach', txt, re.I)
print(match)  # None

None

The string does not string with I like to teach, therefore there was no match and the match method returned None.

1. `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

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')

2. `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()

match.end()

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

match.span()

(2, 8)

3. `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(r'(\d{3}) (\d{2})', re.UNICODE)

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']

['\n', '\r']

Example of RegEx with Metacharacters#

Let us use examples to clarify the meta characters with RegEx methods:

Square Brackets `[]`#

Let us use square bracket to include lower and upper case

# Example 1:

regex_pattern = r'[Aa]pple' # this square bracket mean either A or a
txt = 'Apple and banana are fruits. An old cliche says an apple a day a doctor way has been replaced by a banana a day keeps the doctor far far away.'
matches = re.findall(regex_pattern, txt)
print(matches)  # ['Apple', 'apple']

['Apple', 'apple']

If we want to look for the both apple and banana, we write the pattern as follows:

# Example 2:

regex_pattern = r'[Aa]pple|[Bb]anana' # this square bracket means either A or a
txt = 'Apple and banana are fruits. An old cliche says an apple a day a doctor way has been replaced by a banana a day keeps the doctor far far away.'
matches = re.findall(regex_pattern, txt)
print(matches)  # ['Apple', 'banana', 'apple', 'banana']

['Apple', 'banana', 'apple', 'banana']

Using the square brackets [] and or operator | , we manage to extract Apple, apple, Banana

Escape character `\`#

# Example 1:

regex_pattern = r'\d'  # d is a special character which means digits
txt = "Hawking born on 8 January 1942 and died on 14 March 2018 Einstein's birth anniversary (Pi-Day) and both died at 76"
matches = re.findall(regex_pattern, txt)
print(matches)  # ['8', '1', '9', '4', '2', '1', '4', '2', '0', '1', '8', '7', '6'] - this is not what we want

['8', '1', '9', '4', '2', '1', '4', '2', '0', '1', '8', '7', '6']

One or more times `+`#

# Example 1:

regex_pattern = r'\d+'  # d is a special character which means digits, + mean one or more times
txt = "Hawking born on 8 January 1942 and died on 14 March 2018 Einstein's birth anniversary (Pi-Day) and both died at 76"
matches = re.findall(regex_pattern, txt)
print(matches)  # ['8', '1942', '14', '2018', '76'] - this is better!

['8', '1942', '14', '2018', '76']

Period `.`#

# Example 1:

regex_pattern = r'[a].'  # this square bracket means a and . means any character except new line
txt = '''Apple and Banana are fruits'''
matches = re.findall(regex_pattern, txt)
print(matches)  # ['an', 'an', 'an', 'a ', 'ar']

['an', 'an', 'an', 'a ', 'ar']

# Example 2: [] with +

regex_pattern = r'[a].+'  # . any character, + any character one or more times 
matches = re.findall(regex_pattern, txt)
print(matches)  # ['and Banana are fruits']

['and Banana are fruits']

Zero or more times `*`#

# Example 1:

regex_pattern = r'[a].*'  # . any character, * any character zero or more times 
txt = '''Apple and Banana are fruits'''
matches = re.findall(regex_pattern, txt)
print(matches)  # ['and Banana are fruits']

['and Banana are fruits']

Zero or one time `?`#

Zero or one time. The pattern may not occur or it may occur once.

# Example 1:

txt = '''I am not sure if there is a convention how to write the word e-mail.
Some people write it as email others may write it as Email or E-mail.'''
regex_pattern = r'[Ee]-?mail'  # ? means here that '-' is optional
matches = re.findall(regex_pattern, txt)
print(matches)  # ['e-mail', 'email', 'Email', 'E-mail']

['e-mail', 'email', 'Email', 'E-mail']

Quantifier `{}`#

We can specify the length of the substring we are looking for in a text, using a curly brackets {}. Let us imagine, we are interested in a substring with a length of 4 characters:

# Example 1:

txt = "Hawking born on 8 January 1942 and died on 14 March 2018 Einstein's birth anniversary (Pi-Day) and both died at 76"
regex_pattern = r'\d{4}'  # exactly four times
matches = re.findall(regex_pattern, txt)
print(matches)  # ['1942', '2018']

['1942', '2018']

# Example 2:

txt = "Hawking born on 8 January 1942 and died on 14 March 2018 Einstein's birth anniversary (Pi-Day) and both died at 76"
regex_pattern = r'\d{1,4}'   # 1 to 4
matches = re.findall(regex_pattern, txt)
print(matches)  # ['8', '1942', '76', '14', '2018', '76']

['8', '1942', '14', '2018', '76']

Cart `^`#

# Example 1: Starts with

txt = "Hawking born on 8 January 1942 and died on 14 March 2018 Einstein's birth anniversary (Pi-Day) and both died at 76"
regex_pattern = r'^Hawking'  # ^ means starts with
matches = re.findall(regex_pattern, txt)
print(matches)  # ['Hawking']

['Hawking']

# Example 2: Negation

txt = "Hawking born on 8 January 1942 and died on 14 March 2018 Einstein's birth anniversary (Pi-Day) and both died at 76"
regex_pattern = r'[^A-Za-z ]+'  # ^ in set character means negation, not A to Z, not a to z, no space
matches = re.findall(regex_pattern, txt)
print(matches)  # ['8', '1942', '14', '2018', "'", '(', '-', ')', '76']

['8', '1942', '14', '2018', "'", '(', '-', ')', '76']

Python RegEx

Contents

Python RegEx#

Specify Pattern Using RegEx#

MetaCharacters#

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

1. [] - Square brackets#

2. . - Period#

3. ^ - Caret#

4. $ - Dollar#

5. * - Star#

6. + - Plus#

7. ? - Question Mark#

8. {} - Braces#

9. | - Alternation#

10. () - Group#

11. \ - Backslash#

Special Sequences#

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

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

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

\d - Matches any non-decimal digit. Equivalent to [^0-9]#

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

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

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

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

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