Which regular expression requires backtracking?

Question

There are three different solutions to implement regular expression matching: DFA, NFA and Backtracking. I am looking for examples:

• a regexp, which can be solved w

Answer 1

What I found out so far is:

• Every regular expression, which can be implemented with a NFA, can also be implemented with a DFA. Every NFA can be transformed into a DFA.

• Regular expressions, which require backtracking, are regular expressions, which contain back references like /(a)\1/.

Answer 2

i guess there is more than 1 meaning to the word backtracking - even '.*a' has to backtrack to match the string "lalaiiiiiii" (because .* will first match the whole string - so then a won't match anything - and only then it will give up one character at a time, so the final match would be "lala")

i highly recommend http://www.regular-expressions.info/