TYPO3 Core

This can be achieved easily using e.g. the alphabetic validator. See the following example:

enctype = multipart/form-data
method = post
prefix = tx_form
confirmation = 1
prefix = formPid7
class = formPid7

10 = SELECT
10 {
    label = Select
    name = select

    10 = OPTION
    10 {
        data =
    }

    20 = OPTION
    20 {
        data = Mr
        value = Mr
    }

    30 = OPTION
    30 {
        data = Mrs
        value = Mrs
    }
}

20 = SUBMIT
20 {
    name = submit
}

rules {
    1 = alphabetic
    1 {
        element = select
        message = This field is mandatory.
        error = Please choose an option.
    }
}