regexp? or find # of "."s in string

I currently am testing what level a node is at, based
on the "id" attribute of the node.

ie - 

<tab id="2.3.2.3" state="off">

I currently test for string-length of @id, but if my
numbers go over 10, then the string-length is not a
valid way of determining tab level (ie - 2.1 or 2.1.1,
2.1.10 would break)

I'm currently using:
<xsl:if test="string-length(@id) = '7'">


If I can somehow count the number of periods "." in
the string "id", then I can determine what level the
<tab> element is at...anyone have any ideas how to
count the number of occurences in a string with XSL?

Thanks,
Anthony E.

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