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Java Regex – Quick Guide ”; Previous Next Java Regex – Overview Java provides the java.util.regex package for pattern matching with regular expressions. Java regular expressions are very similar to the Perl programming language and very easy to learn. A regular expression is a special sequence of characters that helps you match or find other strings or sets of strings, using a specialized syntax held in a pattern. They can be used to search, edit, or manipulate text and data. The java.util.regex package primarily consists of the following three classes − Pattern Class − A Pattern object is a compiled representation of a regular expression. The Pattern class provides no public constructors. To create a pattern, you must first invoke one of its public static compile() methods, which will then return a Pattern object. These methods accept a regular expression as the first argument. Matcher Class − A Matcher object is the engine that interprets the pattern and performs match operations against an input string. Like the Pattern class, Matcher defines no public constructors. You obtain a Matcher object by invoking the matcher() method on a Pattern object. PatternSyntaxException − A PatternSyntaxException object is an unchecked exception that indicates a syntax error in a regular expression pattern. Java Regex – Capturing Groups Capturing groups are a way to treat multiple characters as a single unit. They are created by placing the characters to be grouped inside a set of parentheses. For example, the regular expression (dog) creates a single group containing the letters “d”, “o”, and “g”. Capturing groups are numbered by counting their opening parentheses from the left to the right. In the expression ((A)(B(C))), for example, there are four such groups − ((A)(B(C))) (A) (B(C)) (C) To find out how many groups are present in the expression, call the groupCount method on a matcher object. The groupCount method returns an int showing the number of capturing groups present in the matcher”s pattern. There is also a special group, group 0, which always represents the entire expression. This group is not included in the total reported by groupCount. Example Following example illustrates how to find a digit string from the given alphanumeric string − Live Demo import java.util.regex.Matcher; import java.util.regex.Pattern; public class RegexMatches { public static void main( String args[] ) { // String to be scanned to find the pattern. String line = “This order was placed for QT3000! OK?”; String pattern = “(.*)(\d+)(.*)”; // Create a Pattern object Pattern r = Pattern.compile(pattern); // Now create matcher object. Matcher m = r.matcher(line); if (m.find( )) { System.out.println(“Found value: ” + m.group(0) ); System.out.println(“Found value: ” + m.group(1) ); System.out.println(“Found value: ” + m.group(2) ); } else { System.out.println(“NO MATCH”); } } } This will produce the following result − Output Found value: This order was placed for QT3000! OK? Found value: This order was placed for QT300 Found value: 0 Java Regex – MatchResult Interface Introduction The java.util.regex.MatchResult interface represents the result of a match operation. This interface contains query methods used to determine the results of a match against a regular expression. The match boundaries, groups and group boundaries can be seen but not modified through a MatchResult. Interface declaration Following is the declaration for java.util.regex.MatchResult interface − public interface MatchResult Interface methods Sr.No Method & Description 1 int end() Returns the offset after the last character matched. 2 int end(int group) Returns the offset after the last character of the subsequence captured by the given group during this match. 3 String group() Returns the input subsequence matched by the previous match. 4 String group(int group) Returns the input subsequence captured by the given group during the previous match operation. 5 int groupCount() Returns the number of capturing groups in this match result”s pattern. 6 int start() Returns the start index of the match. 7 int start(int group) Returns the start index of the subsequence captured by the given group during this match. Java Regex – Pattern Class Introduction The java.util.regex.Pattern class represents a compiled representation of a regular expression. Class declaration Following is the declaration for java.util.regex.Pattern class − public final class Pattern extends Object implements Serializable Field Following are the fields for java.util.regex.Duration class − static int CANON_EQ − Enables canonical equivalence. static int CASE_INSENSITIVE − Enables case-insensitive matching. static int COMMENTS − Permits whitespace and comments in pattern. static int DOTALL − Enables dotall mode. static int LITERAL − Enables literal parsing of the pattern. static int MULTILINE − Enables multiline mode. static int UNICODE_CASE − Enables Unicode-aware case folding. static int UNICODE_CHARACTER_CLASS − Enables the Unicode version of Predefined character classes and POSIX character classes. static int UNIX_LINES − Enables Unix lines mode. Class methods Sr.No Method & Description 1 static Pattern compile(String regex) Compiles the given regular expression into a pattern. 2 static Pattern compile(String regex, int flags) Compiles the given regular expression into a pattern with the given flags. 3 int flags() Returns this pattern”s match flags. 4 Matcher matcher(CharSequence input) Creates a matcher that will match the given input against this pattern. 5 static boolean matches(String regex, CharSequence input) Compiles the given regular expression and attempts to match the given input against it. 6 String pattern() Returns the regular expression from which this pattern was compiled. 7 static String quote(String s) Returns a literal pattern String for the specified String. 8 String[] split(CharSequence input) Splits the given input sequence around matches of this pattern. 9 String[] split(CharSequence input, int limit) Splits the given input sequence around matches of this pattern. 10 String toString() Returns the string representation of this pattern. Methods inherited This class inherits methods from the following classes − Java.lang.Object Java Regex – Matcher Class Introduction The java.util.regex.Matcher class acts as an engine that performs match operations on a character sequence by interpreting a Pattern. Class declaration Following is the declaration for java.util.regex.Matcher class − public final class Matcher extends Object implements MatchResult Class methods Sr.No Method & Description 1 Matcher appendReplacement(StringBuffer sb, String