"Hi, could I have a grande vanilla iced-coffee?"

"And your name?"

"Zack."

"Is that Zach with an ch or Zack with a ck?"

"You know, I've even seen it done with just a c. But really, isn't this what we have regular expressions for?"

"What?"

"May I?"

The barista hands over the pen and cup, whereupon the customer writes:

/Zac[hk]?/

"There. Now you can't possibly be wrong!"