Advanced Java - IV - Java Regex

Jalaz Kumar · September 7, 2020

Java

API to define a pattern for searching or manipulating strings.

Widely used for defining constraints on strings such as password and email validation.

Java Regex API provides 1 interface and 3 classes in java.util.regex package.

  • MatchResult interface
  • Matcher class
  • Pattern class
  • PatternSyntaxException class

Matcher class

  • Implements the MatchResult interface

  • Regex engine : Used to perform match operations on a character sequence.

  • Following are the important methods:

    • boolean matches()
    • boolean find()
    • boolean find(int start)
    • String group()
    • int start()
    • int end()
    • int groupCount()

Pattern class

  • Compiled version of a regular expression.

  • Used for defining pattern for the regex engine.

  • Following are the important methods:

    • static Pattern compile(String regex)
    • Matcher matcher(CharSequence input)
    • static boolean matches(String regex, CharSequence input)
    • String[] split(CharSequence input)
    • String pattern()
import java.util.regex.*;

public class RegexExample {  
    public static void main(String args[]){  

        Pattern p = Pattern.compile(".s");  
        Matcher m = p.matcher("as");  
        boolean b1 = m.matches();  

        boolean b2=Pattern.compile(".s").matcher("as").matches();  

        boolean b3 = Pattern.matches(".s", "as");  

        System.out.println(b+" "+b2+" "+b3);               // true true true
    }
}

RegEx Essentials

RegEx Character/Symbol Usage & Meaning
^regex match at the beginning of the line
regex$ match at the end of the line
[abc] a, b, or c
[abc][vz] can match a or b or c followed by either v or z
[^abc] Any character except a, b, or c (negation)
[a-zA-Z] a through z or A through Z, inclusive (range)
X|Z Finds X or Z.
XZ Finds X directly followed by Z
X? X occurs once or not at all
X+ X occurs once or more times
X* X occurs zero or more times
X{n} X occurs n times only
. Any character (may or may not match terminator)
\d Any digits, short of [0-9]
\D Any non-digit, short for [^0-9]
\s Any whitespace character, short for [\t\n\x0B\f\r]
\S Any non-whitespace character, short for [^\s]
\S+ Several non-whitespace characters
\w Any word character, short for [a-zA-Z_0-9]
\W Any non-word character, short for [^\w]
a(?!b) (Negative look ahead) match “a” if “a” is not followed by “b”.

The regex is applied on the text from left to right.

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