The following example declares the string terminal commonly used in programming languages. It is declared as a series of zero or more printable characters (not including the double-quotes used for delimiters).
| {String Char} = {Printable} - ["] String = '"'{String Char}*'"' <Literal> ::= Identifier | String |
However, this definition does not allow the programmer to specify the double-quote character. Two distinct approaches are used in modern programming languages. The first approach is to use another structure or constant to represent the double-quote. The second approach is the use of an "override" character that allows the double-quote to be placed directly in the string. The latter approach is used in the C/C++ programming language family.
The following contains a much more complex definition for strings, but implements the second approach mentioned above. The backslash character '\' now acts as an override and can be used to represent the double-quote character. Essentially, the string terminal is now a series of any printable character (not including the double-quote and backslash) and any printable character preceded by the backslash.
| {String Char} = {Printable} - ["\] String = '"' ({String Char} | '\'{Printable})* '"' <Literal> ::= Identifier | String |