Regular Expressions

Regular expressions are powerful tools for pattern matching and text processing. They are represented as a pattern that consists of a special set of characters to search for in a string str. The regex module needs to be imported before use.

This page provides syntax for regular expressions in Python . Each section includes an example to demonstrate the described methods.

Functions

Action	Function
Check if regex matches a string	re.search("pattern", string, flag=0)
Capture regex matches	re.match("pattern", string, flag=0)
Specify alternative regex	pattern1|pattern2

Character Class

Character class specifies a list of characters to match ([...] where ... represents the list) or not match ([^...])

Character Class	...
Any lowercase vowel	[aeiou]
Any digit	[0-9]
Any lowercase letter	[a-z]
Any uppercase letter	[A-Z]
Any digit, lowercase letter, or uppercase letter	[a-zA-Z0-9]
Anything except a lowercase vowel	[^aeiou]
Anything except a digit	[^0-9]
Anything except a space	[^ ]
Any character	.
Any word character (equivalent to [a-zA-Z0-9_])	\w
Any non-word character (equivalent to [^a-zA-Z0-9_])	W
A digit character (equivalent to [0-9])	\d
Any non-digit character (equivalent to [^0-9])	\D
Any whitespace character (equivalent to [\t\r\n\f])	\s
Any non-whitespace character (equivalent to [^\t\r\n\f])	\S

Anchors

Anchors are special characters that can be used to match a pattern at a specified position

Anchor	Special Character
Beginning of line	^
End of line	$
Beginning of string	\A
End of string	\Z

Repetition and Quantifier Characters

Repetition or quantifier characters specify the number of times to match a particular character or set of characters

Repetition	Character
Zero or more times	*
One or more times	+
Zero or one time	?
Exactly n times	{n}
n or more times	{n,}
m or less times	{,m}
At least n and at most m times	{n.m}

Input:

# regex.jl
number1 = "[phone]"
number2 = "123-45-6789"

# check if matches
if occursin(r"\([0-9]{3}\)[0-9]{3}-[0-9]{4}", number1)
   println("match!")
end

if occursin(r"\([0-9]{3}\)[0-9]{3}-[0-9]{4}", number2)
  println("match!")
else
  println("no match!")
end

# capture matches
# use parentheses to "capture" different parts of a regular 
# expression for later use the first set of parentheses corresponds 
# to index 1, second to index 2, etc.

number_details = match(r"\(([0-9]{3})\)([0-9]{3}-[0-9]{4})", number1)

if number_details != nothing
   area_code = number_details[1]
   phone_number = number_details[2]

   println("area code: $area_code")
   println("phone number: $phone_number")
end

Output:

match!
no match!
area code: 555
phone number: 123-4567

Resources

• Regular Expressions 101

• Regular Expressions Library

• Regular Expressions Cheat Sheet

• W3 Schools: Python RegEx