m/
  ^          # anchor to the beginning of the string
  -?         # 0 or 1 dashes (for negative numbers I assume)
  \.?        # 0 or 1 decimal points
  \d+        # 1 or more numbers
  (?:\.\d+)? # 0 or 1 decimal places followed by a number
  $          # anchor to the end of the string
/x

##</code><code>##

# some examples
/^-?\.?\d+/ matches 
    123 or 
    -123 or
    -.123 or
    .123
/^-?\.?\d+(?:\.\d_)?/ matches those and also 
    123.45 or 
    -123.45 or
    -.123.45 (probably a bug!) or
    -123.23532 or
    .235235235235.235235235235235325