nodejs variable in regex match() [duplicate]

This question already has an answer here:

• How do you pass a variable to a Regular Expression JavaScript? 15 answers

how can I use a variable in match() if I want to use regex? This works fine for me:

if (message.match('!' + cmd)) {
  this.client.say(to, myJson[cmd]);
}

but if I try something like this it will not check the variable.

    if (message.match(/!cmd/i)) {
      this.client.say(to, myJson[cmd]);
    }

I already searched for a solution but could only find something for replace.