Question

I need do find all multiline comments in a string and replace them with a space (if the comment is in one line) or with a \n (if the comment is on more than one line).
for example:

int/* one line comment */a;

should be changed to:

int a;

and this:

int/* 
more
than one
line comment*/a;

should be changed to:

int
a;

I have one String with all the text and I used this command:

file = file.replaceAll("(/\\*([^*]|(\\*+[^*/]))*\\*+/)"," ");

where file is the string.

The problem is it finds all multiline comment and I want to separate it to 2 cases.
How can I do it?

Answer 1

This can be solved using Matcher.appendReplacement and Matcher.appendTail.

String file = "hello /* line 1 \n line 2 \n line 3 */"
            + "there /* line 4 */ world";

StringBuffer sb = new StringBuffer();
Matcher m = Pattern.compile("(?m)/\\*([^*]|(\\*+[^*/]))*\\*+/").matcher(file);

while (m.find()) {

    // Find a comment
    String toReplace = m.group();

    // Figure out what to replace it with
    String replacement = toReplace.contains("\n") ? "\n" : "";

    // Perform the replacement.
    m.appendReplacement(sb, replacement);
}

m.appendTail(sb);

System.out.println(sb);

Output:

hello 
there  world

---

Note: If you want to preserve correct line number / columns for all text that is not inside comments (good if you want to refer back to the source code in error messages etc) I would recommend doing

String replacement = toReplace.replaceAll("\\S", " ");

which replaces all non-whitespace with white space. This way \n is preserved, and

"/* abc */"

is replaced by

"         "