let input = "ab<br/><br/>cd<br/>efg<br/><br/><br/>hijk<br/><br/>";
// Regex to match one or more consecutive <br/> tags
let pattern = /(<br\s*\/?>\s*)+/gi;
// Replace with a single <br/>
let output = input.replace(pattern, "<br/>");
console.log(output);
/*
run:
ab<br/>cd<br/>efg<br/>hijk<br/>
*/