XML: Not well-formed (invalid token)

Question

Why this code give me error?

 <clients>
    <client>
     <name!> </name!>
     <cif ></cif>
    </client>
 </clients>

Why can't I use the ! character in an XML element name? Are there any rules against this?

Answer 1

An exclamation point (!) is not permitted by the W3C XML BNF for XML names:

Name          ::= NameStartChar (NameChar)*
NameStartChar ::= ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] |
                  [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] |
                  [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] |
                  [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] |
                  [#x10000-#xEFFFF]
NameChar      ::= NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] |
                  [#x203F-#x2040]