The specification (http://www.w3.org/TR/2004/REC-xmlschema-2-20041028/datatypes.html#regexs) states:

[12] charClassExpr ::= '[' charGroup ']'
[13] charGroup ::= posCharGroup | negCharGroup | charClassSub
[14] posCharGroup ::= ( charRange | charClassEsc )+
[17] charRange ::= seRange | XmlCharIncDash
[18] seRange ::= charOrEsc '-' charOrEsc
[20] charOrEsc ::= XmlChar | SingleCharEsc
[21] XmlChar ::= [^\#x2D#x5B#x5D]
[22] XmlCharIncDash ::= [^\#x5B#x5D]

A single XML character is a ·character range· that identifies the set of characters containing only itself. All XML characters are valid character ranges, except as follows:

    * The [, ], - and \ characters are not valid character ranges;
    * The ^ character is only valid at the beginning of a ·positive character group· if it is part of a ·negative character group·
    * The - character is a valid character range only at the beginning or end of a ·positive character group·.

Note: The grammar for ·character range· as given above is ambiguous, but the second and third bullets above together remove the ambiguity

The regex 'a[-]?c' and '[-]' are valid according to the derivation rules: 12-13-14-17-22 and comments