Regular Expression | Java

Introduction

A regular expression, or regex, is a sequence of characters that specifies a pattern which can be searched for in a text. A regex defines a set of strings, usually united for a given purpose. Suppose you need a way to formalize and refer to all the strings that make up the format of an email address. Since there are a near infinite number of possible email addresses, it’d be hard to enumerate them all. However, as we know an email address has a specific structure, and we can encode that using the regex syntax.

Character Classes

• [abc] : a, b, or c (simple class)
• [^abc] : Any character except a, b, or c (negation)
• [a-zA-Z] : a through z or A through Z, inclusive (range)
• [a-d[m-p]] : a through d, or m through p: [a-dm-p] (union)
• [a-z&&[def]] : d, e, or f (intersection)
• [a-z&&[^bc]] : a through z, except for b and c: [ad-z] (subtraction)
• [a-z&&[^m-p]] : a through z, and not m through p: [a-lq-z](subtraction)

Predefined Character Classes

• .    : Any character (may or may not match line terminators)
• \d : A digit: [0-9]
• \D : A non-digit: [^0-9]
• \h : A horizontal whitespace character
• \H : A non-horizontal whitespace character
• \s : A whitespace character: [ \t\n\x0B\f\r]
• \S : A non-whitespace character: [^\s]
• \v : A vertical whitespace character
• \V : A non-vertical whitespace character
• \w : A word character: [a-zA-Z_0-9]
• \W : A non-word character: [^\w]

Boundary Matchers

• ^ : The beginning of a line
• $ : The end of a line
• \b : A word boundary
• \B : A non-word boundary
• \A : The beginning of the input
• \G : The end of the previous match
• \Z : The end of the input but for the final terminator, if any
• \z  : The end of the input

Greedy Quantifiers

• X?  : X, once or not at all
• X*  :  X, zero or more times
• X+  :  X, one or more times
• X{n}  : X, exactly n times
• X{n,}  : X, at least n times
• X{n,m}  : X, at least n but not more than m times

Logical Operators

• XY : X followed by Y
• X|Y : Either X or Y
• (X) : X, as a capturing group

Precedence

The precedence of character-class operators is as follows, from highest to lowest:

1. Literal escape    \x
2. Grouping       […]
3. Range  a-z
4. Union    [a-e][i-u]
5. Intersection  [a-z&&[aeiou]]

Reference

Class Pattern