[xsl] Getting the offset of a matching-substring in <xsl:analyze-string>

[Martin Holmes <mholmes@xxxxxxx>]

Hi there,

I'm doing some automated markup using <xsl:analyze-string>, and as part 
of the output I'd like to record the offset and length of each matched 
substring. So given input like this in a text node (simplified example):

This is a test.

and a match like this:

  <xsl:analyze-string select="$input" regex="{'[a-zA-Z]+'}">

    <xsl:matching-substring>

      <w offset="???" length="{string-length(.)}">

        <xsl:value-of select="."/>

      </w>

    <xsl>matching-substring>

  </xsl:analyze-string>

I'm wondering if there's any way I can calculate the offset value (which 
for "This" would be 1, for "is" would be 6, etc.).

Does anyone know if there's a way to do this?

All help appreciated,
Martin