master.xml

<data>
  <object id="week_35">Week 35</object>
  <object id="blah_1">Blah 1</object>
  <object id="week_64">Week 64</object>
</data>

master.xsl

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">

  <xsl:output method="xml" indent="yes" />

  <xsl:template match="/">
    <xsl:apply-templates select="data/object[substring(@id, 1, 5) = 'week_']" />
  </xsl:template>

  <xsl:template match="object">
    <p id="{@id}"><xsl:value-of select="." /></p>
  </xsl:template>

</xsl:stylesheet>