[xsl] xsl:analyze-string explanation needed

I'm running a stylesheet to perform a statistical analysis of a series of runs 
on an Ant build.xml file. One of the tasks is to extract the timing information 
from a number of elements in the log file. The timing information is presented 
in an attribute in this format: "3 minutes 57 seconds".

I pass this string as a parameter to three named templates to convert the time 
information into the same units, viz., seconds. Below is one of these 
templates. The problem I have is, apparently, a faulty mental model of the 
functioning of  xsl:analyze-string.

Looking at it, I see the general form as:

Compare the string to the regular expression.
If there is a match, output something.
If there is no match, output something else.

Given the input "3 minutes 57 seconds", I expect the output of this template to 
be "180". Instead, the output is "1800", that is to say, a concatenation of the 
values produced in each of the two branches, rather than one or the other.

Can someone explain the operation of this element and how I can get the output 
of either one OR the other branch, but not both?

Thanks.

<xsl:template name="extract-minutes-as-seconds">
  <xsl:param name="time" />
  <xsl:analyze-string select="$time" regex="([0-9]+ minutes)">
        <xsl:matching-substring>
                <xsl:value-of select="60 * 
xs:integer((substring-before(regex-group(1),' minutes')))" />
        </xsl:matching-substring>
        <xsl:non-matching-substring>
                <xsl:value-of select="'0'" />
        </xsl:non-matching-substring>
  </xsl:analyze-string>
</xsl:template>

-- 
Charles Knell
cknell(_at_)onebox(_dot_)com - email

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