2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(1/8)

Regular Expressions in Java

Joel Adams and Jeremy Frens

Calvin College

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(2/8)

Regular Expression Library

•Java 2 SDK 1.4 introduced the java.util.regex library.

•Regular expression matching added to String class.•Based on the regular expressions as written in Perl.

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(3/8)

Regular Expressions

•Regular expressions are a way to express simple textual patterns in the data.

•Some patterns cannot be recognized by a regular expression (e.g., balanced parentheses).

•Great for verifying input and parameter values.•Great for extracting data from complicated input.

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(4/8)

Building a Regular Expression

• Text matches exactly (e.g., abc).

Character classes

\\s for a whitespace character.\\d for a digit.\\w for a word character (letter, digit,

underscore).[a-z] for any character between a and z.. for any character.

Position ^ the beginning of the string.

$ the end of the string.

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(5/8)

More Buildinga and b are regular expressions; n and m are integers…

a|b matches a or b.

a? matches zero or one as.

a* matches zero or more as.

a+ matches one or more as.

a{n,m} match at least n as and no more than m as.

(a) treats a as a group.

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(6/8)

Simple Matching

•String#matches(String) returns a Boolean indicating if the pattern is found.

String name = “Jeremy”;assertTrue(name.matches(“\\w+”));

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(7/8)

More Powerful Matching

•The Pattern class creates a pattern. A pattern must be compiled. Then it can be matched against real text.

•The Matcher class returns the matches.

Pattern pattern = Pattern.compile(“\\w+”);Matcher matcher = pattern.matcher(“Jeremy”);assertTrue(matcher.find());assertEquals(“Jeremy”, matcher.group());

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(8/8)

Capturing Groups

•Parenthesized groups are also used to capture portions of the match.

•Each captured group is given a number based on the order of the left parentheses: Index 0 is for the whole match. Index 1 is for the first left parenthesis. Index 2 is for the second left parenthesis.Etc…

2003 Jeremy D. Frens. All Rights Reserved. Calvin CollegeDept of Computer Science(9/8)

public class SSN { private String myFirst, myMiddle, myLast; public SSN(String ssn) { Pattern pattern = Pattern.compile( “^(\\d{3})-(\\d{2})-(\\d{4})$”); Matcher matcher = pattern.matcher(ssn); if (!matcher.find()) throw new IllegalArgumentException(); myFirst = matcher.group(1); myMiddle = matcher.group(2); myLast = matcher.group(3); }}

Capture Example