<p>I need a (php) regex to match Yahoo's username rules:</p>

<blockquote>
  <p>Use 4 to 32 characters and start with a letter. You may use letters, numbers, underscores, and one dot (.).</p>
</blockquote>

A one dot limit?  That's tricky.
I'm no regex expert, but I think this would get it, except for that:
```
```
[A-Za-z][A-Za-z0-9_.]{3,31}
```
```
Maybe you could check for the . requirement separately?