----------------------------------- Martin Sat May 28, 2005 2:30 pm Regular Expression to match alternating characters ----------------------------------- This is part of my CS assignment, so I'm looking more for hints than an actual solution. I have to write a regular expression to do the following: All lines in which every other letter is a lowercase a and which contains at least one lowercase a. These a's may be separated by any character (including another a) and the lines may begin with an a or any other character. Thus the lines "a", "aWay", "aha aha" and "Banana" are acceptable, but the lines "b", and "aloha" are not. ----------------------------------- wtd Sat May 28, 2005 3:06 pm ----------------------------------- It's pretty easy. Match an "a" or "A" one time. /a/i Then match any character exactly once or zero times. /a.?/ Now, make this a non-matching group. Now match that group one or more times. ----------------------------------- Martin Sat May 28, 2005 4:22 pm ----------------------------------- Thanks :) Yeah...that was pretty easy. ----------------------------------- wtd Sat May 28, 2005 5:03 pm ----------------------------------- For what it's worth, my solution looks like: /^(?:a.?)+$/i ----------------------------------- rizzix Sat May 28, 2005 5:12 pm ----------------------------------- that won't work... cuz that basically means it has to start with an 'a' ----------------------------------- wtd Sat May 28, 2005 5:31 pm ----------------------------------- The problem description indicated that it does have to start with either an 'a' or 'A'. The one thing I didn't take into account is the string having to contain at least one lowercase 'a'. In this event I'd just do something like: success = str =~ /^(?:a.?)+$/i && str =~ /a/ ----------------------------------- rizzix Sat May 28, 2005 5:41 pm ----------------------------------- i think it says it should start with 'a|A' or any other character... ----------------------------------- wtd Sat May 28, 2005 5:48 pm ----------------------------------- How about: /^(?:.?a)+$/i ----------------------------------- rizzix Sat May 28, 2005 5:53 pm ----------------------------------- that means it has to end with an 'a|A', it will not match "aWay" ----------------------------------- GlobeTrotter Sat May 28, 2005 5:55 pm ----------------------------------- What kind of code is this stuff? I'm pretty confused. ----------------------------------- wtd Sat May 28, 2005 5:59 pm ----------------------------------- that means it has to end with an 'a|A', it will not match "aWay" Hmmm... /^(?:.?a)*.?$/i ----------------------------------- wtd Sat May 28, 2005 6:00 pm ----------------------------------- What kind of code is this stuff? I'm pretty confused. We're talking in regular expressions. [url=http://www.compsci.ca/v2/viewtopic.php?t=6666]What are regular expressions? There are many more tutorials like that one. ----------------------------------- rizzix Sat May 28, 2005 6:07 pm ----------------------------------- that means it has to end with an 'a|A', it will not match "aWay" Hmmm... /^(?:.?a)*.?$/i that means it will match "b" which is wrong. :P ----------------------------------- wtd Sat May 28, 2005 6:31 pm ----------------------------------- Ok smartypants, let's see your answer. :) ----------------------------------- rizzix Sat May 28, 2005 6:36 pm ----------------------------------- /^(?:.?a)+.?$/i still working on checking for at least one 'a'... :P