[Varnish] #1106: Document vcl regex

[Varnish]

#1106: Document vcl regex
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 Reporter:  neaj   |        Type:  documentation
   Status:  new    |    Priority:  normal       
Milestone:         |   Component:  build        
  Version:  3.0.0  |    Severity:  normal       
 Keywords:         |  
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 vcl.7 should mention that varnish uses POSIX regex, together with any
 differences if applicable.

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1106>
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