sub valid_flag {
# ;      Default      -d
# ;      2x2          -2
# ;      2x2BLD       -2b
# ;      3x3          -3
# ;      3x3BLD       -3b
# ;      3x3OH        -3o
# ;      4X4          -4
# ;      4x4BLD       -4b
# ;      4x4OH        -4o
# ;      5x5          -5
# ;      5x5BLD       -5b
# ;      magic        -m
# ;      master magic -mm
# ;      pyraminx     -px
# ;      pyraminx BLD -pxb
# ;      megaminx     -mx
# ;      megaminx BLD -mxb
# ;      square-1     -s
# ;      square-1 BLD -sb
# ;      clock        -c
# ;      cup stacking -cs
  if ($_[0] !~ /^\-([mscxyz]|([2-5s]|[pm]x)b?|[34]o?|mm|cs)$/i) { return 0; } # if the flag entered is not valid, return 0
  $_[0] =~ s/-//;
  $_[0] =~ tr/[A-Z]/[a-z]/;
  return $_[0];  # however, if it is valid, return it without the - and lowercase
}