Regex to replace block comment with line comment

Question

There are tons of examples to do the conversion from C-style line comment to 1-line block comment. But I need to do the opposite: find a regex to replace multi-line block comment with line comments. From:

This text must not be touched
/*
This
is
random
text
*/
This text must not be touched

To

This text must not be touched
// This
// is
// random
// text
This text must not be touched

I was thinking if there's a way to represent "each line" concept in regex, then just add // in front of each line. Something like

\/\*\n(?:(.+)\n)+\*\/ -> // $1

But the greediness nature of the regex engine makes $1 just match the last line before */. I know Perl and other languages have some advanced regex features like recursion, but I need to do this in a standard engine. Is there any trick to accomplish this?

EDIT: To clarify, I'm looking for pure regex solution, not involving any programming language. Should be testable on sites like https://regex101.com/.

What is the environment, in which you can use only standard regular expressions? — Alexander Mashin
– Alexander Mashin, Commented Oct 2, 2020 at 18:19
Use /\/\*([^]*?)\*\//g to match a comment block then replace each ^ with // — anubhava
– anubhava, Commented Oct 2, 2020 at 18:33
So you want to remove the /* */ and prepend the // in one match - replace? — The fourth bird
– The fourth bird, Commented Oct 2, 2020 at 18:51
Yes, Add // in front of each line within the original block, in one match replace. — Reci
– Reci, Commented Oct 2, 2020 at 18:58

Wiktor Stribiżew · Accepted Answer · 2020-10-03 11:09:09Z

2

If you are interested in a single regex pass in the modern JavaScript engine (and other regex engines supporting infinite length patterns in lookbehinds), you can use

/(?<=^(\/)\*(?:(?!^\/\*)[\s\S])*?\r?\n)(?=[\s\S]*?^\*\/)|(?:\r?\n)?(?:^\/\*|^\*\/)/gm

Replace with $1$1, see the regex demo.

Details

(?<=^(\/)\*(?:(?!^\/\*)[\s\S])*?\r?\n) - a positive lookbehind that matches a location that is immediately preceded with
- ^(\/)\* - /* substring at the start of a line (with / captured into Group 1)
- (?:(?!^\/\*)[\s\S])*? - any char, zero or more occurrences, as few as possible, not starting a /* char sequence that appears at the start of a line
- \r?\n - a CRLF or LF ending
(?=[\s\S]*?^\*\/) - a positive lookahead that requires any 0 or more chars as few as possible followed with */ at the start of a line, immediately to the right of the current location
| - or
(?:\r?\n)? - an optional CRLF or LF linebreak
(?:^\/\*|^\*\/) - and then either /* or */ at the start of a line.

edited Oct 3, 2020 at 11:09

answered Oct 2, 2020 at 20:26

Wiktor Stribiżew

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8 Comments

The fourth bird Over a year ago

So you use the capturing group in the lookbehind to be the replacement, because if you use // literally you would get // in the replacement for every match which is too much. And due to the alternation, group 1 can be empty so there is no replacement right?

Wiktor Stribiżew Over a year ago

@Thefourthbird Correct, the solution is tricky, the alternation does not let hardcode the replacement pattern. So, more a hack for this concrete scenario.

The fourth bird Over a year ago

I was playing around to see how to solve it, but this is awesome. One technique to remember :-) ++

Reci Over a year ago

@WiktorStribiżew So if instead we need to prefix something like #/ or @@, this trick won't work, right? We are just lucky then. BTW, the first line in Details is a typo (lookbehind)?

Reci Over a year ago

Problem: if there are /* inside the block comment, they will be replaced as well.

|

Alexander Mashin · Accepted Answer · 2020-10-02 18:44:08Z

1

As usual in such cases, two regular expressions—the second applied to the matches of the first—can do what one cannot achieve.

const txt = `This text must not be touched
/*
This
is
random
text
*/
This text must not be touched`;

const to1line = str => str.replace(
    /\/\*\s*(.*?)\s*\*\//gs,
    (_, comment) => comment.replace( /^/mg, '//')
);
console.log( to1line( txt ));

edited Oct 2, 2020 at 18:44

answered Oct 2, 2020 at 18:33

Alexander Mashin

4,7291 gold badge11 silver badges20 bronze badges

4 Comments

Reci Over a year ago

When I say Javascript, I meant the Javascript style regex. No code is allowed. Can you link a testable regex101.com instance?

Alexander Mashin Over a year ago

I do not think that it can be done with one regular expression, due to variable number of lines, and therefore captures, in a multi-line comment. Even if somebody can, I think he shouldn't.

Reci Over a year ago

I can accept answer like "it is not possible because ..." if you are sure about that. You can keep the existing Javascript code here in case other people can use a code-based solution.

Alexander Mashin Over a year ago

Wait some time, perhaps somebody will find a way.

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