Regex - Using Java Matches

Regex - Using Java Matches

Regex - Using Java Matches

Definition

A regular expression (shortened as regex or regexp; sometimes referred to as rational expression) is a sequence of characters that specifies a search pattern in text. Usually such patterns are used by string-searching algorithms for “find” or “find and replace” operations on strings, or for input validation. https://en.wikipedia.org/wiki/Regular_expression

From Stack Overflow

https://stackoverflow.com/questions/8923398/regex-doesnt-work-in-string-matches

Welcome to Java’s misnamed .matches() method… It tries and matches ALL the input. Unfortunately, other languages have followed suit :(

If you want to see if the regex matches an input text, use a Pattern, a Matcher and the .find() method of the matcher:

Pattern p = Pattern.compile("[a-z]");
Matcher m = p.matcher(inputstring);
if (m.find())
    // match

If what you want is indeed to see if an input only has lowercase letters, you can use .matches(), but you need to match one or more characters: append a + to your character class, as in [a-z]+. Or use ^[a-z]+$ and .find().

Lecture Code

/*
 * Programmer: James Goudy
 * Project: Using Regex to determine valid characters in a string
 */
package com.mycompany.java_regexexpressions;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 *
 * @author jgoudy
 */
public class Java_RegExExpressions {

    public static void main(String[] args) {
        boolean status = true;

        // Test - only want a string expression to only allow 
        // the any capital letters of A B C D,
        // the any lowercase letters of l m n o p,
        // and any numbers of 4 5 6 7 8
        // The regex Expression looks like this ^[A-Dl-p4-8]+$
        
        // Note in order to work correctly in JAVA
        // it must start with the ^ and end with the $
        
        String regex_Expression = "^[A-Dl-p4-8]+$";
        System.out.println("Regex Expression = " + regex_Expression);

        // this string should test false since FF is not allowed
        String testPattern = "AAFFlmno78";
        System.out.println("\nFalse test pattern: " + testPattern);

        Pattern p = Pattern.compile(regex_Expression);
        Matcher m = p.matcher(testPattern);
        status = m.find();
        System.out.println("status = " + status);

        // this pattern should test true
        testPattern = "BBDDoooml55447";
        System.out.println("\nTrue test pattern: " + testPattern);
        
        p = Pattern.compile(regex_Expression);
        m = p.matcher(testPattern);
        status = m.find();
        System.out.println("status = " + status);

        // note that m.find() is a boolean and could be used
        // in if statements, loops, etc.
    }
}

/* Output

Regex Expression = ^[A-Dl-p4-8]+$

False test pattern: AAFFlmno78
status = false

True test pattern: BBDDoooml55447
status = true

 */