Regex to match a C-style multiline comment

The best multiline comment regex is an unrolled version of (?s)/\*.*?\*/ that looks like

String pat = "/\\*[^*]*\\*+(?:[^/*][^*]*\\*+)*/";

See the regex demo and explanation at regex101.com.

In short,

• /\* - match the comment start /*
• [^*]*\*+ - match 0+ characters other than * followed with 1+ literal *
• (?:[^/*][^*]*\*+)* - 0+ sequences of:
  • [^/*][^*]*\*+ - not a / or * (matched with [^/*]) followed with 0+ non-asterisk characters ([^*]*) followed with 1+ asterisks (\*+)
• / - closing /

David's regex needs 26 steps to find the match in my example string, and my regex needs just 12 steps. With huge inputs, David's regex is likely to fail with a stack overflow issue or something similar because the .*? lazy dot matching is inefficient due to lazy pattern expansion at each location the regex engine performs, while my pattern matches linear chunks of text in one go.

Try using this regex (Single line comments only):

String src ="How are things today /* this is comment */ and is your code /* this is another comment */ working?";
String result=src.replaceAll("/\\*.*?\\*/","");//single line comments
System.out.println(result);

REGEX explained:

Match the character "/" literally

Match the character "*" literally

"." Match any single character

"*?" Between zero and unlimited times, as few times as possible, expanding as needed (lazy)

Match the character "*" literally

Match the character "/" literally

Alternatively here is regex for single and multi-line comments by adding (?s):

//note the added \n which wont work with previous regex
String src ="How are things today /* this\n is comment */ and is your code /* this is another comment */ working?";
String result=src.replaceAll("(?s)/\\*.*?\\*/","");
System.out.println(result);

Reference:

• https://www.regular-expressions.info/examplesprogrammer.html