Following case:
An Existing field without certain length (could contain whitespaces). I need ever the last 14 characters:

<data>
<invoice>
  <field >testString</field ></invoice>
</data>

Well
  <xsl:template match="field">
    <xsl:value-of select="substring(., string-length(.) - 13)"/>
  </xsl:template>
should do, as far as I understand the requirement.