Removing non-alphanumeric characters from string in XSL

If you define non-alphanumeric as [^a-zA-Z0-9]:

<xsl:value-of select="
  translate(
    string,
    translate(
      string, 
      'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789',
      ''
    ),
    ''
  )
" />

Note that this is for XSLT 1.0. In XSLT 2.0 you can work with regexes directly, using replace().

For XSLT 2.0 you can use replace() as follows:

<xsl:value-of select="replace(<string>, '[^a-zA-Z0-9]', '')" />