I'm new to ER and I have the following exercise to solve:
In PL/I, a string is enclosed in double quotes. But if inside the string also has a quotation mark, it must be preceded by another quotation mark. So, the string
The quotation mark, ", should be typeset in italics
would be written in PL/I as
“The quotation mark, ““, should be typeset in italics”
Design an ER to recognize strings in PL/I. Assume that the strings contain only symbols defined in an alphabet Σ and that quotes are the only special case.
I can already capture the string in quotes with:
^["].*.["]$
But I can't write the conditional for "if the quotes exist, there must be a quote before"
I've searched a lot, but haven't found a solution yet. Any help is appreciated.