[xsl] XPath to check that a middle initial is either empty or is a lette

Hi Folks,

I need an XPath expression that returns true if the <MI> element is empty or, 
if it's not empty, it contains an uppercase letter of the English alphabet 
followed by a period. For example, this <MI> element is good:

        <MI></MI>

As is this:

        <MI>H.</MI>

This XPath seems to work fine:

        if (MI eq '') then true()
        else if (exists(MI)) then matches(MI, '^[A-Z]\.$') 
        else true()

Is there a better (shorter, simpler, more robust) XPath expression?

/Roger
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