Today, I am going to discuss my attempts to parse c style comments.
For example,
//This is a comment
/***This is also
*** a comment ***/
Initially, I came up with a regex for /*…*/ style comments :
/\*.*\*/
Well, the above expression was not able to parse comments like:
/*** This is a comment ***/
I googled and came across http://ostermiller.org/findcomment.html where I found the regex:
/\*(.|[\r\n])*?\*/
This was able to match comments like the above one. But it’d also match the following /*…*/ comments which are not really comments:
s = "This is a string: /* with a comment */";
//comment1 /*
foo();
//comment2 */
I then worked on a regex for //… style comments: //[^\n]*\n
Then I combined the two regexes by or and my regex pattern becomes:
//[^\n]*\n|/\*(.|[\r\n])*?\*/
Now, this pattern is able to search for both: //… and /*…*/ style comments and avoid matches for patterns like:
//comment1 /*
foo();
//comment2 */
One caveat that remains is the /*…*/ pattern in
s = "This is a string: /* with a comment */";
getting matched. If any one has a work around this issue, please comment.
I hope this helps.
This should work:
(?s)\/\*(?:(\*(?!/))|(?:[^\*]))*\*/|//[^\n\r]*(?=[\n\r])
(?s) tells the . to match newlines characters too
the rest of the regex means:
/* followed by ( or ) repeated any number of times, followed by */
OR
// followed by
(?!) is a negative lookahead
(?=) is a positive lookahead
The positive lookahead is useful to avoid deleting newlines after comments. Please tell me if I missed something, some exercise is always good
It is still able to parse a comment pattern in a C string
, e.g.:
s = “This is a string: /* with a comment */”;
A work around could be to combine the comment regex with other regexes for elements in the language, so that the comment pattern inside a string would not be matched by the comment regex.
Let’s take a simple example:
ws = regex for whitespace
cs = above regex for comment
var = regex for valid variable
str = regex for a string
So, we can combine these into a new pattern:
(ws|cs)*var(ws|cs)*=(ws|cs)*str(ws|cs)*;(ws|cs)*
This is a very crude example. But, it works. Now the pattern knows, where to search for comments and where not to.