Regardless of the language/platform, your RE pattern will have three parts:

• mark the start of the string as a capture group: @[
• match the middle of the string: Abdullah
• mark the rest of the string as a capture group: ](61dfaceadef99a95503119a8)

Assuming you're just looking for the regex pattern in ECMAScript, you'll need something like:

(\@\[).*(\]\([a-f0-9]+\))

Run that through match() or matchAll() to identify matches, or use replace() on the matches. javascript.info has an example using capture groups - that looks close enough to your scenario.

I finally had time to get it done and came up with an answer also thanks to @Denton Thomas answer i was able to understand what to do.

let str = "hey @[Abdullah](61dfaceadef99a95503119a8) check this out and @[James](61dfaceadef99a95514562) welcome to the group!";

let regexp = /((.)\[([^[]*)]\(([^(^)]*)\))/gi 
var result = str.replace(regexp, function(data){
  console.log("data:", data)
  let name = data.match(/([^[]+(?=]))/g)
  console.log("name:", name)
  return name[0]
});

console.log("message:", result );