java regular expression help

Question

I am trying to replace all the characters between an html font tag with an expression. I wrote a little test program but it is not working correcty. Here is my regular expression:

test.replaceAll("<font\b(.*)>", "Something");

This does not work.

Why?

Answer 1

You probably want two "\" before the "b":

test.replaceAll("<font\\b(.*)>", "Something");

You need this because the regular expression is a string and backslashes need to be escaped in strings.

To make it only match up to the first ">", do this:

test.replaceAll("<font\\b(.*?)>", "Something");

This makes the * "lazy", so that it matches as little as possible rather than as much as possible.

However, it seems that it is better to write this particular expression as follows:

test.replaceAll("<font\\b([^>]*)>", "Something");

This has the same effect and avoids backtracking, which should improve performance.

Answer 2

Note that the * operator is greedy, i.e.,

String test = "<font size=\"10\"><b>hello</b></font>";
System.out.println(test.replaceAll("<font\\b(.*)>", "Something"));

prints

Something

You may want to use [^>]*

test.replaceAll("<font\\b([^>]*)>", "Something")

or a reluctant quantifier, *?:

test.replaceAll("<font\\b(.*?)>", "Something")

which both result in

Something<b>hello</b></font>