There's a problem with using {2,10} in the same line since it has a comma. You need to put the pattern into a variable and use that instead.

alias checksteam {
  var %r = /^STEAM_[0-1]:[0-1]:\d{2,10}$/
  return $regex($1-,%r)

Returns 1 if valid and 0 if not. There's certainly no need to use an if-else here.

if (1) and if ($true) are essentially the same, as are if (0) and if ($false).

So, using this in another script:

if ($checksteam(id)) will be false if it returns 0 and true if it returns 1.