Pattern Matching with Multiple Regex in Scala

Question

I am attempting to make a lexer in Scala. I am attempting to do something like this

def lex(s: String): Expr = s match {
  case num(a)    => Number(a.toDouble)
  case mul(a, b) => Mul(Number(a.toDouble), Number(b.toDouble))
  case div(a, b) => Div(Number(a.toDouble), Number(b.toDouble))
  case add(a, b) => Add(Number(a.toDouble), Number(b.toDouble))
  case sub(a, b) => Sub(Number(a.toDouble), Number(b.toDouble))
  case   _       => Number(0)
}

where num, mul, div, add, sub are defined as so:

val num: Regex = "[0-9]+".r
val add: Regex = "[0-9]+\\s*\\+\\s*[0-9]+".r
val sub: Regex = "[0-9]+\\s*\\-\\s*[0-9]+".r
val div: Regex = "[0-9]+\\s*\\/\\s*[0-9]+".r
val mul: Regex = "[0-9]+\\s*\\*\\s*[0-9]+".r

When attempting to lex any expression (lex("1 + 2")) the result is always Number(0.0) instead of the expected Add(Number(1), Number(2)) Im not sure where it's going wrong...

You can use the raw interpolator to avoid having to use double back-slashes. For example: raw"[0-9]+\s*\+\s*[0-9]+".r — Hosam Aly
– Hosam Aly, Commented Oct 19, 2018 at 16:33

Sebastian Celestino · Accepted Answer · 2018-10-19 15:56:48Z

4

You need to specify which groups you want to extract.

val num = "([0-9]+)".r
val add = "([0-9]+)\\s*\\+\\s*([0-9]+)".r
val sub = "([0-9]+)\\s*\\-\\s*([0-9]+)".r
val div = "([0-9]+)\\s*\\/\\s*([0-9]+)".r
val mul = "([0-9]+)\\s*\\*\\s*([0-9]+)".r

You need one pair of parentheses per variable you extract.

answered Oct 19, 2018 at 15:56

Sebastian Celestino

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