Excellent! With this:

matches(flt:Aircraft/flt:Altitude, '[0-9]+')

I can do datatype checking, without using XML Schemas.

This only determines that a value is castable to a subset of values of
the xs:integer type.

The real type of the value may be quite different, for example the xml
schema associated with this xml document may define it to be an
xs:string, and the fact that this instance just happens to be castable
to an xs:integer may be accidental.

We have the same in XSLT 1.0 -- for example an <xsl:sort/> instruction
will sort all sort key values as strings (regardless of the fact that
all they may be interpretable as numbers) unless explicitly directed
otherwise by specifying the attribute

--
Cheers,
Dimitre Novatchev
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Truly great madness cannot be achieved without significant intelligence.
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To invent, you need a good imagination and a pile of junk
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You've achieved success in your field when you don't know whether what
you're doing is work or play