Question

I just have a question about Regex…. I really have no idea so any kind of help is appreciated.

The question is: Define a regex that characterises the language consisting of all strings of a’s and b’s in which the 2nd and 2nd-Last characters are ‘b’.

Any ideas on how to start the question?

Thanks very much

Answer 1

To solve this you need to:

1. find a string that starts with an a or b: ^(a|b),
2. has a b as the second character `^(a|b)b
3. is followed by a string of as, bs or both: ^(a|b)b[a-b]*
4. has another b: ^(a|b)[a-b]*b
5. and ends with another a or b: ^(a|b)b[a-b]*b(a|b)$
6. then you need to test your string against that regular expression: /^(a|b)b[a-b]*b(a|b)$/.test(string)

A proof of concept using jQuery and JavaScript:

​$('li').each(
    function(i,el) {
        var text = $(el).text();
        if (/^((a|b)b[a-b]*b(a|b))$/.test(text)) {
            $(el).addClass('matched');
        }
    });​​​

JS Fiddle demo.

References:

• Regular Expressions, at the MDN.