You can do this because regular expressions define extractors but you need to define the regex pattern first. I don't have access to a Scala REPL to test this but something like this should work.

val Pattern = "([a-cA-C])".r
word.firstLetter match {
   case Pattern(c) => c bound to capture group here
   case _ =>
}

As delnan pointed out, the match keyword in Scala has nothing to do with regexes. To find out whether a string matches a regex, you can use the String.matches method. To find out whether a string starts with an a, b or c in lower or upper case, the regex would look like this:

word.matches("[a-cA-C].*")

You can read this regex as "one of the characters a, b, c, A, B or C followed by anything" (. means "any character" and * means "zero or more times", so ".*" is any string).

String.matches is the way to do pattern matching in the regex sense.

But as a handy aside, word.firstLetter in real Scala code looks like:

word(0)

Scala treats Strings as a sequence of Char's, so if for some reason you wanted to explicitly get the first character of the String and match it, you could use something like this:

"Cat"(0).toString.matches("[a-cA-C]")
res10: Boolean = true

I'm not proposing this as the general way to do regex pattern matching, but it's in line with your proposed approach to first find the first character of a String and then match it against a regex.

EDIT: To be clear, the way I would do this is, as others have said:

"Cat".matches("^[a-cA-C].*")
res14: Boolean = true

Just wanted to show an example as close as possible to your initial pseudocode. Cheers!

To expand a little on Andrew's answer: The fact that regular expressions define extractors can be used to decompose the substrings matched by the regex very nicely using Scala's pattern matching, e.g.:

val Process = """([a-cA-C])([^\s]+)""".r // define first, rest is non-space
for (p <- Process findAllIn "aha bah Cah dah") p match {
  case Process("b", _) => println("first: 'a', some rest")
  case Process(_, rest) => println("some first, rest: " + rest)
  // etc.
}