Regular Expressions in Java

In the project of Data Mining, I have to make use of the regular expressions to deal with the large amount of text in html.

I used regular expression in Linux (grep) before and find it quite an efficient way to deal with text, especially when their amount is very large.

Introduction

Regular expressions are a way to describe a set of strings based on common characteristics shared by each string in the set. They can be used to search, edit, or manipulate text and data. You must learn a specific syntax to create regular expressions — one that goes beyond the normal syntax of the Java programming language. Regular expressions vary in complexity, but once you understand the basics of how they're constructed, you'll be able to decipher (or create) any regular expression.

The package of java.util.regex

It primary consists three classes:

Pattern: a compiled representation of a regular expression.

Matcher: interprets the Patten and performs match operation against an input string.

PatternSyntaxException: indicates an syntax error in a regular expression pattern

A single regular expression program

package regexTestHarness;

import java.util.regex.Pattern;
import java.util.regex.Matcher;
import java.io.BufferedReader;
import java.io.InputStreamReader;

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

            System.out.println("%nEnter your regex: ");

            InputStreamReader isr = new InputStreamReader(System.in);

            BufferedReader br = new BufferedReader(isr);

            String s = br.readLine();

            Pattern pattern = Pattern.compile(s);

            System.out.println("%nEnter your text: ");

            isr = new InputStreamReader(System.in);

            br = new BufferedReader(isr);

            s = br.readLine();

            Matcher matcher = pattern.matcher(s);

            boolean found = false;
            while (matcher.find()) {
                System.out.print("I found the text " + matcher.group()
                        + " starting at " + "index " + matcher.start()
                        + " and ending at index " + matcher.end());
                found = true;
            }
            if (!found) {
                System.out.println("No match found.");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

}

 

Chracter classes and Predefined classes

Construct	Description
[abc]	a, b, or c (simple class)
[^abc]	Any character except a, b, or c (negation)
[a-zA-Z]	a through z, or A through Z, inclusive (range)
[a-d[m-p]]	a through d, or m through p: [a-dm-p] (union)
[a-z&&[def]]	d, e, or f (intersection)
[a-z&&[^bc]]	a through z, except for b and c: [ad-z] (subtraction)
[a-z&&[^m-p]]	a through z, and not m through p: [a-lq-z] (subtraction)

Construct	Description
.	Any character (may or may not match line terminators)
\d	A digit: [0-9]
\D	A non-digit: [^0-9]
\s	A whitespace character: [ \t\n\x0B\f\r]
\S	A non-whitespace character: [^\s]
\w	A word character: [a-zA-Z_0-9]
\W	A non-word character: [^\w]

 

Quantifiers

Greedy	Reluctant	Possessive	Meaning
X?	X??	X?+	X, once or not at all
X*	X*?	X*+	X, zero or more times
X+	X+?	X++	X, one or more times
X{n}	X{n}?	X{n}+	X, exactly n times
X{n,}	X{n,}?	X{n,}+	X, at least n times
X{n,m}	X{n,m}?	X{n,m}+	X, at least n but not more than m times

Chinese Characters

[\u4e00-\u9fa5]