JSFiddle - React, Tailwind, and code Playground

HTML

<textarea style=width:100%;height:300px></textarea>

JavaScript

let str = `.a {
    // rules
}

.a-b,
.a-b .b, .a-b.s
.x .y, .x {
    // rules
}

@a {
    // rules
}

@k {
    // rules
}`;
let edited = str.replace(/^[\*\.#@:\-\[][\s\S]+?(?=\{)/mg, match => {
	return match.replace(/\n/g, ' ');
});
document.querySelector('textarea').value =edited;