Question

I have the following class PlaceHolderConverter used to parse string like "my {} are beautiful" into strings with filled variables.

For instance new PlaceHolderConverter("\\{\\}").format("my {} are beautiful", "flowers") will return the string "my flowers are beautiful".

package something;


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

public class PlaceHolderConverter
{
    public Pattern lookForVar;

    public PlaceHolderConverter( String placeHolder )
    {
        this.lookForVar = Pattern.compile( placeHolder );
    }

    public String format( String text, String... args )
    {
        if ( args == null || args.length == 0 )
        {
            return text;
        }
        StringBuffer stringBuffer = new StringBuffer();
        Matcher matcher = lookForVar.matcher( text );
        short varCount = 0;
        while ( matcher.find() )
        {
            matcher.appendReplacement( stringBuffer, args[varCount++] );
        }
        matcher.appendTail( stringBuffer );
        return stringBuffer.toString();
    }
}

As you can see in the following tests, I have issue with the special character dollar because it’s a special character for java regex.
I tried to solve that with Pattern.quote() but without results.

package something;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

import java.util.regex.Pattern;

public class PlaceHolderConverterTest
{
    private PlaceHolderConverter placeHolderConverter;

    @Before
    public void before()
    {
        placeHolderConverter = new PlaceHolderConverter( "\\{\\}" );
    }

    @Test // SUCCESS
    public void whenStringArgsThenReplace()
    {
        String result = placeHolderConverter.format( "My {} are beautifull", "flowers" );
        Assert.assertEquals( "My flowers are beautifull", result );
    }

    @Test // FAIL IllegalArgumentException illegal group reference while calling appendReplacement
    public void assertEscapeDollar()
    {
        String result = placeHolderConverter.format( "My {} are beautiful", "flow$ers" );
        Assert.assertEquals( "My flow$ers are beautiful", result );
    }

    @Test // FAIL IllegalArgumentException illegal group reference while calling appendReplacement
    public void assertEscapeDollarWithQuote()
    {
        String result = placeHolderConverter.format( "My {} are beautiful", Pattern.quote("flow$ers") );
        Assert.assertEquals( "My flow$ers are beautiful", result );
    }

}

I also to tried to manually escape the dollar before using it in regexp, with something like .replaceAll("\\$", "\\\\$") but it seems that replaceAll dislike to have to arg1 included in arg2.

How I can fix that ?

Patch can be provided here https://gist.github.com/3937872

Answer 1

When replacing a fixed string, there is no need to call the regex methods on String because there is a simpler one: input.replace("$", "\\$");. With this method you won’t have any of the troubles caused by the special meaning of the dollar sign, and it will be (very slightly) faster as a bonus.