Using Regular Expressions in Java

for Data Validation

Evelyn Brannock

Jan 30, 2009

Definition

They are usually used to give a concise description of a set, without having to list all elements

A regular expression is an expression that describes a set of strings

Origin In the 1950s, mathematician Stephen Cole

Kleene devised a way to describe and classify formal languages using his mathematical notation called regular sets

Using Regular Expressions for Data Validation

Regular Expressions

Construct Matchesx The character x

[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)

\d A digit: [0-9]

\D A non-digit: [^0-9]

\s A whitespace character

\S A non-whitespace character: [^\s]

\w A word character: [a-zA-Z_0-9]

\W A non-word character: [^\w]

*from the Sun Java API

Regular Expressions

Construct MatchesX? 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

XY X followed by Y

X|Y Either X or Y

*from the Sun Java API

Examples

Searching for a wordMatching an email addressVerifying the format of a telephone numberCredit card pre-validation IP address determinationZip code format validation

The regular expression built is determined by the definition of valid values for the field

Building a Regular Expression**Matching a Word**

Words that start with t, with at least one additional letter, all lowercase

t[a-z]+What if the word can start with a

lowercase or uppercase t? [t|T][a-z]+What if no additional letters must follow

(but they may)?

Building a Regular Expression **Validating and IP Address**

What does an IP address look like?Are three digits required? Is 999.999.999.999 a valid IP?25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?

Steps

Whitespace will often need to be strippedDetermine the valid values of the stringCreate the regular expressionCompile the regular expression into a

patternPlace the string you wish to validate into

the Matcher classDoes the string match the pattern?

Java Invocation Sequence

Two Java classes required:

import java.util.regex.Pattern;

import java.util.regex.Matcher;

Code:

Pattern p = Pattern.compile("a*b"); Matcher m = p.matcher("aaaaab"); boolean b = m.matches();

U.S. Zip Code

What is a valid expression for a zip code?Let’s build the regular expression together

using GGCCustomerZipCode.java

Further Reading

Teach Yourself Regular Expressions in 10 Minutes by Ben Forta

Mastering Regular Expressions by Jeffrey Friedl Java Regular Expressions by Mehran Habibi Regular Expression Pocket Reference by Tony

Stubblebine Regular Expression Recipes by Nathan Good

Questions