Regex to match a C-style multiline comment

Question

I have a string for e.g.

String src = \"How are things today /* this is comment *\\*/ and is your code  /*\\* this is another comment */ working?\"


        

Answer 1

Try this one:

(//[^\n]*$|/(?!\\)\*[\s\S]*?\*(?!\\)/)

If you want to exclude the parts enclused in " " then use:

(\"[^\"]*\"(?!\\))|(//[^\n]*$|/(?!\\)\*[\s\S]*?\*(?!\\)/)

the first capturing group identifies all " " parts and second capturing group gives you comments (both single line and multi line)

copy the regular expression to regex101 if you want explanation

Answer 2

System.out.println(src.replaceAll("\\/\\*.*?\\*\\/ ?", ""));

You have to use the non-greedy-quantifier ? to get the regex working. I also added a ' ?' at the end of the regex to remove one space.

Answer 3

This could be the best approach for multi-line comments

System.out.println(text.replaceAll("\\/\\*[\\s\\S]*?\\*\\/", ""));

Answer 4

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

Answer 5

Try this which worked for me:

System.out.println(src.replaceAll("(\/\*.*?\*\/)+",""));

Answer 6

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.