Question

I come from a Perl background and am used to doing something like the following to match leading digits in a string and perform an in-place increment by one:

my $string = '0_Beginning';

$string =~ s|^(\d+)(?=_.*)|$1+1|e;

print $string;        # '1_Beginning'

With my limited knowledge of Java, things aren’t so succinct:

String string = "0_Beginning";

Pattern p = Pattern.compile( "^(\\d+)(?=_.*)" );

String digit = string.replaceFirst( p.toString(), "$1" ); // To get the digit

Integer oneMore = Integer.parseInt( digit ) + 1;          // Evaluate ++digit

string.replaceFirst( p.toString(), oneMore.toString() );  //

The regex doesn’t match here… but it did in Perl.

What am I doing wrong here?

Answer 1

Actually it matches. You can find out by printing

System.out.println(p.matcher(string).find());

The issue is with line

String digit = string.replaceFirst( p.toString(), "$1" );

which is actually a do-nothing, because it replaces the first group (which is all you match, the lookahead is not part of the match) with the content of the first group.

You can get the desired result (namely the digit) via the following code

Matcher m = p.matcher(string);
String digit = m.find() ? m.group(1) : "";

Note: you should check m.find() anyways if nothing matches. In this case you may not call parseInt and you’ll get an error. Thus the full code looks something like

Pattern p = Pattern.compile("^(\\d+)(?=_.*)");

String string = "0_Beginning";

Matcher m = p.matcher(string);
if (m.find()) {
    String digit = m.group(1);
    Integer oneMore = Integer.parseInt(digit) + 1;
    string = m.replaceAll(oneMore.toString());
    System.out.println(string);
} else {
    System.out.println("No match");
}