Wednesday, June 27, 2012

Regular Defination


We may wish to give to regular expression and to define regular expression using these names as if they were symbols.                                                                           
If ∑ is an alphabet of basic symbols, then a regular definition is a sequence of definitions of the form,                                                                                                       
d1 --> r1                                                                                                                           
d2 --> 2                                                                                                                            
  …….                                                                                                                                 
dn --> rn
Where each di is a distinct name, and each ri is a regular expression over the symbols in  ∑ u{d1,d2,….dn}, ie. the basic symbols and the previously defined names. By restricting each ri to symbols of  ∑ and the previously defined names, we can construct a regular expression over ∑  for any ri by repeatedly replacing regular expression names by the expression they denote. If ri used dj for some j>=i, then ri might be recursively defined .and this substitution process would not terminate.                                                                                
To distinguish names from symbols, we print the names in regular definitions in bold face.

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