xml – Can JSON field names start with a number?

Short answer: JSON field names may begin with a number; XML element names may not.

Details

JSON has no restrictions unique to the first character of a member string,

member ::= ws string ws ':' element

string ::= '"' characters '"'

characters ::= "" | character characters

character ::= '0020' . '10FFFF' - '"' - '' | '' escape

but XML does not allow tag names to begin with a number:

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]

Therefore, you’ll have to restrict your JSON to not begin with a number if you wish there to be an unmodified mapping of JSON keys to XML tag names.