Replace a substring by a changing replacement string

Question

I'm trying to write a small program that detect comments in a code file, and tag them by a index-tag, meaning a tag with an increasing value.
For example this input:

method int foo (int y) { 
    int temp; // FIRST COMMENT
    temp = 63; // SECOND COMMENT
    // THIRD COMMENT
}

should be change to:

method int foo (int y) { 
    int temp; <TAG_0>// FIRST COMMENT</TAG>
    temp = 63; <TAG_1>// SECOND COMMENT</TAG>
    <TAG_2>// THIRD COMMENT</TAG>
}

I tried the following code:

    String prefix, suffix;
    String pattern = "(//.*)";

    Pattern r = Pattern.compile(pattern);
    Matcher m = r.matcher(fileText);

    int i = 0;
    suffix = "</TAG>";

    while (m.find()) {
        prefix = "<TAG_" + i + ">";
        System.out.println(m.replaceAll(prefix + m.group() + suffix));
        i++;
    }

The output for the above code is:

method int foo (int y) { 
    int temp; <TAG_0>// FIRST COMMENT</TAG>
    temp = 63; <TAG_0>// SECOND COMMENT</TAG>
    <TAG_0>// THIRD COMMENT</TAG>
}

Answer 1

To replace occurrences of detected patterns, you should use the Matcher#appendReplacement method which fills a StringBuffer:

StringBuffer sb = new StringBuffer();
while (m.find()) {
    prefix = "<TAG_" + i + ">";
    m.appendReplacement(sb, prefix + m.group() + suffix);
    i++;
}
m.appendTail(sb); // append the rest of the contents

The reason replaceAll will do the wrong replacement is that it will have the Matcher scan the whole string to replace every matched pattern with <TAG_0>...</TAG>. In effect, the loop would only execute once.