[xsl] Using Tokenize with Lookbehind Regexp

[Guenstig und Gut <guenstig-und-gut@xxxxxx>]

Hi,

I'm using Saxon 9.0.0.6 and want to use the XPath tokenize function as 
follows:

<xsl:value-of select="tokenize( . , '\s(?&lt;=(.){70,}?)' )"/>
or
<xsl:value-of select="tokenize( . , '\s(?<=(.){70,}?)' )"/>
or
<xsl:value-of select="tokenize( . , '(\s(?<=(.){70,}?))' )"/>

This throws following error:

Position: 815:0
Description: Error at character 3 in regular expression 
"\s(?<=(.){70,}?)": expected ())
URL: http://www.w3.org/TR/2005/WD-xpath-functions-20050211/#ERRFORX0002

I didn't get an error if I use
<xsl:value-of select="tokenize( . , '[\s(?&lt;=(.){70,}?)]' )"/>
But i'll get another result

Can somebody tell me how to use lookbehinds (and lookahaed)?

I want to separate the string at each white-space if the string before 
is longer than 70 signs

Best regards
Walter