Build a regex to match a given integer number range

BEGIN {
  MAX=2000
  for (a = 0; a <= MAX; a++) {
    for (b = a; b <= MAX; b++) {
      print "\n"a"-"b
      regex = range2regex(a,b)
      split(regex,r,"|")
      for (i = 1; i in r; i++) print r[i]
    }
  }
}

function range2regex(a,z,    k,m,b,r) {
  k = 10
  while (a <= z) {
    while (a > 0 && (m = a%k) == 0) k *= 10
    b = a - m + k - 1
    if (b > z) {
     k /= 10
     break
    }
    r = (r ? r "|" : "") range2charclass(a,b)
    a = b + 1
    k *= 10
  }
  while (a <= z) {
    while ((b = z - (z+1)%k) < a) k /= 10
    r = (r ? r "|" : "") range2charclass(a,b)
    a = b + 1
    k /= 10
  }
  return r
}

function range2charclass(a,b,    k,p,q,r) {
  do {
    p = substr(a,++k,1)
    q = substr(b,k,1)
    r = r (p==q ? p : (p+1==q ? "["p""q"]" : "["p"-"q"]"))
  } while (p != "")
  return r
}