Schema Regular Expression which subtracts characters

["Roger L. Costello" <costello@mitre.org>]

Problem: create an XML Schema regular expression which produces strings
with the characters a-z, A-Z minus the two characters o ("oh") and l
("el").

Examples.  
   The regular expression should accept these strings:

         quick, hum, trade

   It should reject these strings:

         john, family  (john contains "o", and 
                        family contains "l")

Below is my regular expression.  It uses the "character class
substraction" capability from the XML Datatype's spec to subtract-out
the two undesired characters.  Here is the regular expression:

    ([a-zA-Z]+)-[ol]

"All the characters a-z and A-Z, one or more times, minus the two
characters o, and l."

Is this correct?  I tried it out with xerces 1.4.1 and also with XML Spy
1.4b and both bombed.  Here's an error message from xerces:

[Error] test.xml:6:26: Datatype error: Value 'quick' does not match
regular expression facet '([a-xA-Z]+)-[ol]'

Perhaps I am using the "character class substraction" capability
incorrectly?  Can someone tell me where I err?   /Roger