Regular Expression

Introduction

Regular Expression, is a sequence of characters that forms a search pattern.

RegEx can be used to check if a string contains the specified search pattern.

RegEx

Metacharacters

Metacharacters are characters with a special meaning:

Character	Description	Example
[]	A set of characters	“[a-m]”
\	Signals a special sequence (can also be used to escape special characters)	“\d”
.	Any character (except newline character)	“he..o”
^	Starts with	“^hello”
$	Ends with	“planet$”
*	Zero or more occurrences	“he.*o”
+	One or more occurrences	“he.+o”
?	Zero or one occurrences	“he.?o”
{}	Exactly the specified number of occurrences	“he{2}o”

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Special Sequences

A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:

Character	Description	Example
\A	Returns a match if the specified characters are at the beginning of the string	“\AThe”
\b	Returns a match where the specified characters are at the beginning or at the end of a word (the “r” in the beginning is making sure that the string is being treated as a “raw string”)	r”\bain” r”ain\b”
\B	Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the “r” in the beginning is making sure that the string is being treated as a “raw string”)	r”\Bain” r”ain\B”
\d	Returns a match where the string contains digits (numbers from 0-9)	“\d”
\D	Returns a match where the string DOES NOT contain digits	“\D”
\s	Returns a match where the string contains a white space character	“\s”
\S	Returns a match where the string DOES NOT contain a white space character	“\S”
\w	Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character)	“\w”
\W	Returns a match where the string DOES NOT contain any word characters	“\W”

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Sets

A set is a set of characters inside a pair of square brackets [] with a special meaning:

Set	Description
[arn]	Returns a match where one of the specified characters (a, r, or n) are present
[a-n]	Returns a match for any lower case character, alphabetically between a and n
[^arn]	Returns a match for any character EXCEPT a, r, and n
[0123]	Returns a match where any of the specified digits (0, 1, 2, or 3) are present
[0-9]	Returns a match for any digit between 0 and 9
[0-5][0-9]	Returns a match for any two-digit numbers from 00 and 59
[a-zA-Z]	Returns a match for any character alphabetically between a and z, lower case OR upper case
[+]	In sets, +, *, ., `

RegEx Module

Python has a built-in package called re, which can be used to work with Regular Expressions.

The re module offers a set of functions that allows us to search a string for a match:

Function	Description
findall	Returns a list containing all matches
search	Returns a Match object if there is a match anywhere in the string
split	Returns a list where the string has been split at each match
sub	Replaces one or many matches with a string

Greedy Matching

RegEx does a greedy match by default. This means that the matchmaking will be as long as possible. Check out the example below. It refers to any match that ends in r and can be any character preceded by it. But it does not stop at the first letter r.

ber beer beeer beeeer

RegEx: .*r matching all the text

Lazy Matching

Lazy matchmaking, unlike greedy matching, stops at the first matching. For example, in the example below, add a ? after * to find the first match that ends with the letter r and is preceded by any character. It means that this match will stop at the first letter r.

ber beer beeer beeeer

RegEx: .*?r matching the first word.