EBNF is used to describe the formal grammar of a language. A rule has the form 'symbol ::= expression'. If a symbol is defined by a regular expression then it has an initial capital letter, otherwise it has an initial lower case letter

	
#xN      N is a hexadecimal integer. The value 
         is a character in ISO/IEC 10646
'string' matches a literal string that inside
"string" the single and double quotes
[a-z]    matches any character from a to z
[^a-z]   matches any character outside the 
         ranage a to z
[^abc]   matches any character not among the 
         given character
(expr)   is a unit
A?       is optional A; matches A or nothing
A*       matches zero or more occurences of A
A+       matches one or more occurences of A
A B      matches A followed by B
A|B      matches A or B but not both
A-B      matches A but does not match B