In the Microsoft regex tests, test RegexTest_24 has the following in the schema:

<xs:pattern value='{5'/>

The metadata describes this as valid.

This reveals a problem in the specification. The specification says in the text:

(1) An atom is either a ·normal character·, (or other options that aren't relevant here)

(2) A normal character is any XML character that is not a metacharacter. 

(3) A metacharacter is either ., \, ?, *, +, {, } (, ), [ or ].

This would imply that the character "{" cannot be used without escaping, making the above regex invalid.

However, the grammar has:

[10]   	Char ::= [^.\?*+()|#x5B#x5D]

which permits "{" and "}" without escaping, making the above regex valid.