How to Concatenate a Regular Expression

To concatenate a regular expression in JavaScript, you can use a combination of the + operator and the RegExp() class as shown below. You need to combine both the RegExp source (the string representation of the RegExp) and flags (options for the RegExp).

let reg = /mastering/g;
let exp = /js/i;
let pattern = concatRegexp(reg, exp);
let string = 'masteringjs';
pattern.test('masteringjs'); // true

function concatRegexp(reg, exp) {
  let flags = reg.flags + exp.flags;
  flags = Array.from(new Set(flags.split(''))).join();
  return new RegExp(reg.source + exp.source, flags);
}

You are responsible for removing duplicate flags. If you pass a duplicate flag to new RegExp(), JavaScript will throw a SyntaxError: Invalid flags.

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