validate.r

rebol []

num: charset [ #"0" - #"9" #"." ]

view layout [
    f1: field
    f2: field

    button "valdate" [
        either parse f1/text [ some num end ][
            if error? err: try [
                decimal? load f1/text
            ][
                alert "Not decimal value in field"
            ]
        ][
            alert "Non numeric characters in field"
        ]
    ]
]