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?" 

I want to remove /* this is comment *\*/ and /** this is another comment */ substrings from the src string.

I tried to use regex but failed due to less experience.

Answer 1

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.

Answer 2

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