How to improve the regex performance in java

I have this code to convert the whole text that is before "=" to uppercase

Matcher m = Pattern.compile("((?:^|\n).*?=)").matcher(conteudo);
while (m.find()) {
  conteudo = conteudo.replaceFirst(m.group(1), m.group(1).toUpperCase());
}

But when the string is too large, it becomes very slow, I want to find a faster way to do that.

Any sugestions?

EDIT

I haven't explained right. I have a text like this

field=value
field2=value2
field3=value3

And I want to convert each line like this

FIELD=value
FIELD2=value2
FIELD3=value3

Solution

The fastest way to get regex to work fast is to not use regex. Regex was never meant to be and almost never is a good choice for performance-sensitive operations. (Further reading: Why are regular expressions so controversial?)

Try using String class methods instead, or write a custom method doing what you want. Use a tokenizer with split on '=', and then use .toUpperCase() on the tailing part (what's after \n). Alternatively, just convert to char[] or use charAt() and traverse it manually, switching chars to upper after a newline and back to regular way after '='.

For example:

public static String changeCase( String s ) {
    boolean capitalize = true;
    int len = s.length();
    char[] output = new char[len];
    for( int i = 0; i < len; i++ ) {
      char input = s.charAt(i);
      if ( input == '\n' ) {
        capitalize = true;
        output[i] = input;
      } else if ( input == '=' ) {
        capitalize = false;
        output[i] = input;
      } else {
        output[i] = capitalize ? Character.toUpperCase(input) : input;
      }
    }
    return new String(output);
}

Method input:

field=value\n
field2=value2\n
field3=value3

Method output:

FIELD=value\n
FIELD2=value2\n
FIELD3=value3

Try it here: http://ideone.com/k0p67j

PS (by Jamie Zawinski):

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.