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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