or if you prefer, since you are not generating elements you don't really
need analyze-string and can just do it in xpath:

<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
>


<xsl:template name="main">
  <xsl:for-each select="(
			'3 minutes 57 seconds',
			'3 minutes',
			'5 seconds',
			'rubbish')">

:<xsl:value-of select="."/>
:::: <xsl:value-of select="60*number(concat('0',replace(.,'([0-9]+) minutes.*$|.+$','$1')))"/>
  </xsl:for-each>
  
</xsl:template>

</xsl:stylesheet>

$ saxon8 -it main regex.xsl 
<?xml version="1.0" encoding="UTF-8"?>

:3 minutes 57 seconds
:::: 180

:3 minutes
:::: 180

:5 seconds
:::: 0

:rubbish
:::: 0