 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 Building a and b are regular expressions; n and m are integers… a|ba|b matches a or b. a?a? matches zero or one a s. a*a* matches zero or more a s. a+a+ matches one or more a s. a{n,m}a{n,m} match at least n a s and no more than m a s. (a)(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