# 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