btw, there is one site to play with RegExp http://erik.eae.net/playground/regexp/regexp.html. Have fun.
a brief introduction can be found from here http://en.wikipedia.org/wiki/Regular_expression. On Wed, Jun 3, 2009 at 4:01 PM, Wu JIANG <albert.w.ji...@gmail.com> wrote: > actually, a+ means at least one 'a', b? means zero or one 'b'. > > > On Wed, Jun 3, 2009 at 10:56 AM, hugo rivera <uai...@gmail.com> wrote: > >> Hello, >> I am experimenting with some regexp implementations (namely the one >> from "the practice of programming") and I am a little disoriented by >> the use of the '?' operator in plan 9's grep: >> say I have the following input >> >> aaaabbb >> ab >> aaaab >> bb >> b >> aaabb >> aaaa >> >> which I feed into grep with >> >> grep 'a+bb?' >> >> which should match at least one 'a' followed by one or two 'b'. So, >> grep's output is >> >> aaaabbb >> ab >> aaaab >> aaabb >> >> which really surprised me at first, since I wasn't expecting the first >> line. After some thought, I realized that the 'aaaab' and the 'aaaabb' >> patterns, contained in the first line of input, match the regexp, so >> grep prints the line. >> But then, how exactly the '?' operator is useful for grep? I was >> thinking that it was good to filter lines that contain more characters >> that desired, but it is not. >> Saludos >> -- >> Hugo >> >> >
