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What are non-permissible characters in XML?

 There aren’t many non-permissible characters in XML, but should you use one then it can cause serious problems with your data.  The main problems that arise from the use of non-permissible characters is often associated with incorrect usage of punctuation.

As mentioned previously, periods can’t be used in the naming of XML elements, along with hyphens and colons.  The use of these characters makes the program or browser that’s working with the XML document think that something else is going on… either you’re listing a filename, you’re attempting to perform an advanced function within an element name, or you’re assigning a namespace to the element that hasn’t been defined.  In any of these instances, you’ll end up getting an error returned.

Other characters that can’t be used in the naming of elements for (hopefully) obvious reasons include <, >, [, ], {, }, as well as the comma itself.  As a general rule, it’s best to avoid punctuation except for where it’s part of the code or contained within an element, or else some generally bad and unwanted effects can occur.