Relax NG does not support end-of-string regular expression (dollar)

I need to give some limited possibilities for an Attribute in xml, for this I am using in Relax NG validation document:

<param "pattern">myRegEx</param>

allowing to specify some regular expression the attribute need to be conform to.

For some reason pattern does not support use of $ (end-of-string) sign.

Example:

<param "pattern">Why it does not work??$</param>

This is supposed to match to string ending with "Why it does not work??" sentence. But the $ seems to be totally ignored from the validation.

Does anyone understand why?

thank you

Nathaniel