Question

In some implementations of “regular expressions,” the notations \1, \2, and so on can occur in...

In some implementations of “regular expressions,” the notations \1, \2, and so on can occur in a search pattern. For example, consider the search pattern ^([a-zA-Z]).*\1$. Here, \1 represents a recurrence of the same substring that matched [a-zA-Z], the part of the pattern between the first pair of parentheses. The entire pattern, therefore, will match a line of text that begins and ends with the same letter. Using this notation, write a pattern that matches all strings in the language L = {a n ban | n ≥ 0}. (Later in this chapter, we will see that L is not a regular language, so allowing the use of \1 in a “regular expression” means that it’s not really a regular expression at all! This notation can add a real increase in expressive power to the patterns that contain it.)

Homework Answers

Answer #1

Note that here, I have used some general rules that are abided by the students who learn regular expressions, besides the rules given in the question.

Also, note that I have tried to explain some of those rules above.

Please do an upvote, if it helps you.
Happy learning.

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