Here's one way to do it:

/a+b*/

For that matter, when you want to match one or more a's, you know that the match will succeed when you find even the first one. So, /a/ will match the same set of strings as the first two. The description "any string containing at least one a followed by any number of b's" means the exact same thing as "any string containing a." Of the sample strings, this matches all of them except fred.

There are even more ways to make this pattern than we show here. Often, in trying to write a pattern, you will need to decide which one of many possible patterns best suits your needs.

Here's one way to do it:

/\\*\**/

And what about the sample strings? Did it match any of them? You bet: it matches all of them! It's because the backslashes and asterisks aren't required in the pattern; that is, this pattern can match the empty string. Here's a rule you can rely upon: when a pattern may freely match the empty string, it'll always match, since the empty string can be found in any string. In fact, it'll always match in the first place that you look.

So, this pattern matches all four characters in \\**, as you'd expect. It matches the empty string at the beginning of fred, which you may not have expected. In the string barney \\\***, it matches the empty string at the beginning. You might wish it would hunt down the backslashes and stars at the end of that string, but it doesn't bother. It looks at the beginning, sees zero backslashes followed by zero asterisks, declares the match a success, and goes home to watch television. And in *wilma\, it matches just the star at the beginning; as you see, this pattern never gets away from the beginning of the string, since it always matches at the first opportunity.

Now, if someone asked you for a pattern to match any number of backslashes followed by any number of asterisks, you'd be technically correct to give them this one. But chances are, that's not what they really wanted. Spoken languages like English may be ambiguous and not say exactly what they mean, but regular expressions always mean exactly what they say they mean.

In this case, maybe the person who asked for the pattern forgot to say that he or she always wants to match at least one character, when the pattern matches at all. We can do that. If there's at least one backslash, /\\+\**/ will match. (That's just like what we had before, but there's a plus in place of the first star, meaning one or more backslashes.) If there's not at least one backslash, then in order to match at least one character, we'll need at least one asterisk, so we want /\*+/. When you put those two possibilities together, you get:

/\\+\**|\*+/

Ugly, isn't it? Regular expressions are powerful but not beautiful. And they've contributed to Perl being maligned as a "write-only language." To be sure that no one criticizes your code in that way, though, it's kind to put an explanatory comment near any pattern that's not obvious. On the other hand, when you've been using these for a year, you will have a different definition of "obvious" than you have today.

How does this new pattern work with the sample strings? With \\**, it matches all four characters, just like the last one. It won't match fred, which is probably the right behavior given the problem description. For barney \\\***, it matches the six characters at the end, as you hoped. And for *wilma\, it matches the asterisk at the beginning.

Here's one way to do it:

while (<>) {
  if (/wilma/) {
    print;
  }
}

This is a grep-like program. For each line of text (contained in $_), we check to see whether the pattern matches. If it matches, we print it. This program uses print's default: if you don't tell it to print something else, it prints $_. So we have written a program that uses $_ all the way through, but never mentions it anywhere. Perl folks love to use the defaults and save time typing, so you'll see a lot of programs that do this.

Here's one way to do it:

while (<>) {
  if (/wilma/) {
    if (/fred/) {
      print;
    }
  }
}

This tests /fred/ only after we find /wilma/ matches, but fred could appear before or after wilma in the line; each test is independent of the other.