Regular Expression to match alternating characters

[Martin]

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]

It's pretty easy.

Match an "a" or "A" one time.

code:

/a/i

Then match any character exactly once or zero times.

code:

/a.?/

Now, make this a non-matching group. Now match that group one or more times.

[Martin]

Thanks

Yeah...that was pretty easy.

[wtd]

For what it's worth, my solution looks like:

code:

/^(?:a.?)+$/i

[rizzix]

that won't work... cuz that basically means it has to start with an 'a'

[wtd]

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:

code:

success = str =~ /^(?:a.?)+$/i && str =~ /a/

[rizzix]

i think it says it should start with 'a|A' or any other character...

[wtd]

How about:

code:

/^(?:.?a)+$/i

[rizzix]

that means it has to end with an 'a|A', it will not match "aWay"

[GlobeTrotter]

What kind of code is this stuff? I'm pretty confused.

[wtd]

rizzix wrote:

that means it has to end with an 'a|A', it will not match "aWay"

Hmmm...

code:

/^(?:.?a)*.?$/i

[wtd]

GlobeTrotter wrote:

What kind of code is this stuff? I'm pretty confused.

We're talking in regular expressions.

What are regular expressions?

There are many more tutorials like that one.

[rizzix]

wtd wrote:

rizzix wrote:

that means it has to end with an 'a|A', it will not match "aWay"

Hmmm...

code:

/^(?:.?a)*.?$/i

that means it will match "b" which is wrong.

[wtd]

Ok smartypants, let's see your answer.

[rizzix]

PerlRegex:

/^(?:.?a)+.?$/i

still working on checking for at least one 'a'...