How to use back reference in Java regex

Question

Question for the RE experts: Consider the following Perl script:

my @lines = (
        "Once upon a time in a galaxy far, far away, there lived\n",
        "this _idiot_ trying to _mark up_ a few lines of\n",
        "marked down text using yet another _language_.\n");

foreach (@lines) {
        s|_(.+?)_|<em>$1</em>|g;
        print
}

The output of % perl [aboveScript] is

Once upon a time in a galaxy far, far away, there lived
this <em>idiot</em> trying to <em>mark up</em> a few lines of
marked down text using yet another <em>language</em>.

I am trying to achieve this in Java. The class I have come up with follows. It works and I get the same output as above, but I am pretty sure this is not the way to do this. My question - how would you implement the "parseLine()" method?

import java.util.*;
import java.util.regex.*;

public class Reglob {

        private final static Pattern emPattern = Pattern.compile ("_(.+?)_");

        public void parseLine (String[] lines) {
                for (String line : lines) {
                        List<Integer>   bList = new ArrayList<Integer>(),
                                        eList = new ArrayList<Integer>();
                        Matcher m = emPattern.matcher (line);
                        int n = 0;
                        while (m.find()) {
                                // System.out.println ("Match indices: " + m.start() + ", " + m.end());
                                bList.add (m.start());
                                eList.add (m.end());
                                n++;
                        }
                        if (n == 0) {
                                System.out.println (line);
                        } else {
                                String s = line.substring (0, bList.get(0));
                                for (int i = 0 ; i < n-1 ; i++) {
                                    s += "<em>"
                                        + line.substring(1+bList.get(i),eList.get(i)-1)
                                        + "</em>" + line.substring (eList.get(i), bList.get(i+1));
                                }
                                s += "<em>"
                                        + line.substring(1+bList.get(n-1),eList.get(n-1)-1)
                                        + "</em>" + line.substring (eList.get(n-1), line.length());
                                System.out.println (s);
        }}}

        public static void main (String[] args) {
                String[] lines = {
                        "Once upon a time in a galaxy far, far away, there lived",
                        "this _idiot_ trying to _mark up_ a few lines of",
                        "marked down text using yet another _language_."};
                new Reglob().parseLine (lines);
}}

possible duplicate of [Java; String replace (using regular expressions)?](http://stackoverflow.com/questions/632204/java-string-replace-using-regular-expressions) — 200_success, Aug 22 '14 at 05:55
The question is only for experts to answer? I feel discriminated... — Unihedron, Aug 22 '14 at 06:01
The dupe by @200_success was answered by tofu beer. How have I missed tofu beer? There's another answer by Can Berk Güder, which sounds like another exotic brew, maybe Swedish, and a good one. — som-snytt, Aug 22 '14 at 06:03
Anybody other than myself in Java regex is an expert - I am only a novice. Now I got it... so many correct answers, wondering which one to pick! I do not even need a pattern for this - oh, well! — Manidip Sengupta, Aug 22 '14 at 06:07

score 2 · Accepted Answer · answered Aug 22 '14 at 05:56

This is the Java equivalent of your Perl script:

public class Main {
    public static void main(String[] args) {
        String[] lines = {
                "Once upon a time in a galaxy far, far away, there lived\n",
                "this _idiot_ trying to _mark up_ a few lines of\n",
                "marked down text using yet another _language_.\n" };

        for(String line : lines) {
            String output = line.replaceAll("_(.+?)_", "<em>$1</em>");

            System.out.print(output);
        }
    }
}

It outputs:

Once upon a time in a galaxy far, far away, there lived
this <em>idiot</em> trying to <em>mark up</em> a few lines of
marked down text using yet another <em>language</em>.

Avinash Raj · Answer 2 · 2014-08-22T06:03:27.810

1

You could simply do like this,

String [] s = {  "Once upon a time in a galaxy far, far away, there lived",
                "this _idiot_ trying to _mark up_ a few lines of",
                 "marked down text using yet another _language_."};
for(String s2 : s)
{
System.out.println(s2.replaceAll("_([^_]+)_", "<em>$1</em>"));
}

edited Aug 22 '14 at 06:03

answered Aug 22 '14 at 05:56

Avinash Raj

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score 0 · Answer 3 · answered Aug 22 '14 at 05:56

Try something like this:

public static final String str = "Once upon a time in a galaxy far, far away, there lived\n" +
        "this _idiot_ trying to _mark up_ a few lines of\n" +
        "marked down text using yet another _language_.\n";

public static void main(String[] args) {
    System.out.println(str.replaceAll("_(.+?)_", "<em>$1</em>"));
}

The output for this:

Once upon a time in a galaxy far, far away, there lived
this <em>idiot</em> trying to <em>mark up</em> a few lines of
marked down text using yet another <em>language</em>.

score 0 · Answer 4 · answered Aug 22 '14 at 05:59

Similarly, in Scala:

scala> val text = """Once upon a time in a galaxy far, far away, there lived
     | this _idiot_ trying to _mark up_ a few lines of
     | marked down text using yet another _language_."""
text: String =
Once upon a time in a galaxy far, far away, there lived
this _idiot_ trying to _mark up_ a few lines of
marked down text using yet another _language_.

scala> val r = "_(.+?)_".r
r: scala.util.matching.Regex = _(.+?)_

scala> r.replaceAllIn(text, """<em>$1<\em>""")
res3: String =
Once upon a time in a galaxy far, far away, there lived
this <em>idiot<em> trying to <em>mark up<em> a few lines of
marked down text using yet another <em>language<em>.

I try everything out in the Scala REPL first, then convert to Java.

But since you have plenty of Java answers, I'll return to Hulu.

How to use back reference in Java regex

4 Answers4