Empty input in analyze-string

Hello,

  I cannot figure out how to tell xsl:analyze-string that empty input
should not match regex.

eg.

    <xsl:variable name="myregex">^([CF]\.[0-9\.]+)\s*(.*)$</xsl:variable>
    <xsl:analyze-string select="$para2" regex="{$myregex}">
      <xsl:matching-substring>
        <xsl:value-of select="regex-group(1)"/>
      </xsl:matching-substring>
...

if para2 is an empty element, then the regex success and return an
empty string. How do I express in my regex that empty is a
non-matching regex ?

Thanks !
-- 
Mathieu