how to escape single quote in xslt substring function

The general rules for escaping are:

In 1.0:

• if you want the attribute delimiter in a string literal, use the XML
  escape form " or '
• if you want the string delimiter in a string literal, you’re hosed

In 2.0:

• if you want the attribute delimiter in a string literal, use the XML
  escape form " or '
• if you want the string delimiter in a string literal, double it (for
  example, ‘I can”t’)

The use of a variable $quot or $apos as shown by Vitaliy can make the code much clearer.