optest = getRecordHead("u_OperatorName")

repeat
    // Start assuming we're good, in each repeat of this loop.
    checkStr = true
    
    // Loop through the character array (string) character by character
    //   - logical AND between true and the result of isAlphaNumeric on the current character
    //   - if it's ever false, there's an invalid character, ask for input
    for chr_index = 1 to len(optest)
        checkStr = checkStr and getParameterNamed("optest", chr_index).isAlphaNumeric
    next chr_index
    
    if len(optest) == 0 then
        checkStr = false
    endif
    
    // If an invalid character was found, prompt for input. Could also be: checkStr == false
    if not checkStr then
        message("Operator Name Invalid.")
        optest = inquireText("Please enter your Stamp ID. Ex: M000")
        setRecordHead("u_OperatorName", optest)
        
        // If no input was given, cancel run completely. (Also break out of possibly infinite loop.)
        if (len(optest) == 0) or (optest == "") then
            message("Input not entered. Cancelling run.")
            cncBreak()
        endif
    endif
until checkStr