Regular Expressions

A regular expression (regex) is a sequence of characters that defines a search pattern. Regular expressions are used for pattern matching in strings — finding, validating, and extracting text that matches a specific pattern. They are part of OCR H446 section 1.4.

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Why Regular Expressions?

Use case	Example
Validation	Check if an email address is in the correct format
Searching	Find all phone numbers in a document
Extraction	Pull dates out of a block of text
Replacement	Replace all instances of "colour" with "color"
Lexical analysis	Identify tokens in source code (numbers, identifiers, keywords)

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Basic Metacharacters

Metacharacter	Meaning	Example	Matches
.	Any single character (except newline)	a.c	"abc", "a1c", "a c"
*	Zero or more of the preceding element	ab*c	"ac", "abc", "abbc", "abbbc"
+	One or more of the preceding element	ab+c	"abc", "abbc", "abbbc" (NOT "ac")
?	Zero or one of the preceding element	ab?c	"ac", "abc" (NOT "abbc")
^	Start of string	^Hello	"Hello world" (matches), "Say Hello" (no match)
$	End of string	world$	"Hello world" (matches), "world class" (no match)
		Alternation (OR)	cat

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Character Classes

Character classes match one character from a defined set:

Syntax	Meaning	Example	Matches
[abc]	Any one of a, b, or c	[aeiou]	Any single vowel
[a-z]	Any character from a to z (range)	[a-z]	Any lowercase letter
[A-Z]	Any uppercase letter	[A-Z]	"A", "B", ..., "Z"
[0-9]	Any digit	[0-9]	"0", "1", ..., "9"
[a-zA-Z]	Any letter (upper or lower)	[a-zA-Z]	Any letter
[^abc]	Any character EXCEPT a, b, or c	[^0-9]	Any non-digit character

Key Term: The ^ inside square brackets means "NOT" — it negates the character class. Outside square brackets, ^ means "start of string."

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Quantifiers

Quantifiers specify how many times a preceding element can occur:

Quantifier	Meaning	Example	Matches
*	0 or more	a*	"", "a", "aa", "aaa", ...
+	1 or more	a+	"a", "aa", "aaa", ... (NOT "")
?	0 or 1	a?	"", "a"
{n}	Exactly n times	a{3}	"aaa"
{n,}	n or more times	a{2,}	"aa", "aaa", "aaaa", ...
{n,m}	Between n and m times	a{2,4}	"aa", "aaa", "aaaa"

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Grouping and Alternation

Syntax	Meaning	Example	Matches
(abc)	Group — treats "abc" as a single unit	(ab)+	"ab", "abab", "ababab"
a	b	Alternation — "a" OR "b"	cat
(cat	dog)s	Alternation within a group	(cat

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Common Regex Patterns