Introduction to Regular Expressions in Java
In Java, Regex or Regular Expression is an Application Program Interface that helps define a pattern to search, manipulate, and edit strings. Java regular expressions are widely used in the validation of passwords and emails. These expressions are provided by java.util.regex package and consists of 1 interface and 3 classes.
The three classes are:
- Pattern: Helps in defining patterns.
- Matcher: Using patterns helps in performing match operations.
- PatternSyntaxException: Helps in indicating a syntax error.
Java Regex has one interface known as MatchResultInterface that helps in determining the regular expression’s match operation result.
Syntax of Regular Expression in Java
Let us see how to write a regular expression in Java with the help of a program.
Code:
//Java program to demonstrate regular expressions
import java.util.regex.*;
public class RegExamples {
public static void main(String args[]){
String A = " Happiness is " + " within yourself";
String B = ".*within.*";
// checks whether the string A contains the word 'within' and stores the result in matchcheck
boolean matchcheck = Pattern.matches(B, A);
//prints the result
System.out.println("Is there any string 'within' in the text ? \n " + matchcheck);
} }Output:
Commonly Used Methods in Regular Expressions
There are 3 commonly used methods in regular expressions.
1. Index Methods
Index methods offer index values that help in precisely showing where the match was found in the string given as input.
| Method | Description |
| start() | The previous match’s start index is returned. |
| start(int group) | Given the group’s previous match operation, the subsequence is captured and returned. |
| end() | The offset after matching the last character is returned. |
| End(int group) | Given the group’s previous match operation, subsequence is captured and offset after matching its last character returned. |
2. Study Methods
Study methods check the string given as input, and a Boolean value is returned, indicating whether the pattern is found or not.
| Method | Description |
| lookingAt() | Match the sequence given as input against the pattern from the beginning of the region. |
| find() | Finds the next subsequence of the sequence given as input against the pattern from the beginning of the region. |
| find(int start) | Resets the matcher and then finds the next subsequence of the sequence given as input against the specified index pattern. |
| matches() | Matches content against the pattern. |
3. Replacement Methods
Methods that are used to replace text in a string.
| Method | Description |
| appendReplacement(StringBuffer s, String replacement) | A non-terminal append and replacement step will be implemented. |
| appendTail(StringBuffer s) | A terminal append and replacement step will be implemented. |
| replaceAll(String replacement) | Replace all subsequence of the sequence given as input that matches against the pattern with a replacement string. |
| quoteReplacement(String s) | A literal replacement string will be returned for the mentioned string. |
| replaceFirst(String replacement) | Replace the first subsequence of the sequence given as input that matches the pattern with a replacement string. |
How to Define Regular Expression in Java?
There are several ways in which a regular expression can be defined.
1. Literals
Suppose a string “hai” has to be searched in the text “hai”.
It can be done using syntax.
Pattern.matches("hai", "hai")2. Character Classes
It matches every single character in the text given as input against multiple permitted characters in the character class.
The following are the various class constructs.
| Character Class | Explanation |
| [pqr] | Matches the text if it contains either p, q or r, and it should be only once. |
| [^pqr] | ^ denotes the negation, and due to that, here, single character except for p, q, or r are taken. |
| [a-zA-Z] | a to z and A to Z are considered. |
| [a-d[p-s]] | a to d, or p to s. |
| [a-dm-p] | Union of both ranges. |
| [a-z&&[pqr]] | a to z and (p, q or r). |
| [a-z&&[^pq]] | a to z and also, p, q are not considered. |
| [ad-z] | Performs the subtraction. |
| [a-z&&[^m-p]] | a to z and not m to p. |
3. Metacharacters
Metacharacters act like shortcodes in the regular expression.
The following are some of the metacharacters commonly used.
| Regular Expression | Explanation |
| \d | Any digit from 0 to 9. It can be written as [0-9] as well. |
| \D | Any non-digit from 0 to 9. It can be written as [^0-9] as well. |
| \s | Whitespace character or [\t\n\x0B\f\r]. |
| \S | Non whitespace character or [^\s]. |
| \w | Word character or [a-zA-Z_0-9]. |
| \W | Non-word character or [^\w]. |
| \b | Word boundary. |
| \B | Non-word boundary. |
4. Quantifiers
Quantifiers mention the count of occurrence of each character to match against the string.
| Regular Expression | Explanation |
| a? | It occurs once or not at all. |
| A* | A occurs 0 or more times. |
| A+ | A occurs 1 or more times. |
| A{n} | A occurs exactly n times. |
| A{n,} | A occurs n or more than that. |
| A{n,m} | A occurs at least n times, but it should not be more than m times. |
How to Create Regular Expression in Java?
Now, let us see a java program with the above-mentioned regular expressions.
Code:
//Java program to demonstrate regular expressions
import java.util.regex.*;
public class RegExamples {
public static void main(String args[]){
String str="hai";
// Returns true if string 1 matches string 2
System.out.println("Returns true if 'hai' matches 'Hai' :"+
Pattern.matches(str, "Hai")); //False
//Returns true if Hai or hai matches parameter 2
System.out.println("Returns true if 'Hai' or 'hai' matches 'Hai' : "+
Pattern.matches("[Hh]ai", "Hai")); //True
// Returns true if the string matches exactly "ann" or "Ann" or "jak" or "Jak"
System.out.println("Returns true if the string matches exactly 'ann' or 'Ann' or 'jak' or 'Jak' with 'Ann' : "+
Pattern.matches("[aA]nn|[jJ]ak", "Ann"));//True
//returns true if the string contains "with" at any place in the string
System.out.println("returns true if the string contains 'with' in the string 'within' : " +
Pattern.matches(".*with.*", "within"));//True
// returns true if the '9448anna' does not have number in the beginning
System.out.println( "returns true if the '9448anna' does not have number in the beginning : "+
Pattern.matches("^[^\\d].*", "9448anna")); //False
System.out.println("returns true if the '9448anna' does not have number in the beginning : " +
Pattern.matches("^[^\\d].*", "anna9448")); //True
}
}Output:
Conclusion – Regular Expressions in Java
Java Regular Expressions are widely used for real-time applications such as password and email verification. These expressions are APIs that define patterns and offer searching, editing, and several other operations in the string.
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