Mastering Regular Expressions - Table of Contents

Matching an Email Address
Page 294
I'd like to finish with a lengthy example that brings to bear many of the regex
techniques seen in these last few chapters, as well as some extremely valuable
lessons about building up a complex regular expression using variables. Verifying
correct syntax of an Internet email address is a common need, but unfortunately,
because of the standard's complexity, * it is quite difficult to do simply. In fact, it
is impossible with a regular expression because address comments may be nested.
(Yes, email addresses can have comments: comments are anything between
parentheses.) If you're willing to compromise, such as allowing only one level of
nesting in comments (suitable for any address I've ever seen), you can take a stab
at it. Let's try.
Still, it's not for the faint at heart. In fact, the regex we'll come up with is 4,724
bytes long! At first thought, you might think something as simple as
\w+\@[.\w]+ could work, but it is much more complex. Something like
Jeffy
is perfectly valid as far as the specification is concerned.** So, what constitutes a
lexically valid address? Table 7-11 lists a lexical specification for an Internet
email address in a hybrid BNF/regex notation that should be mostly
self-explanatory. In addition, comments (item 22) and whitespace (spaces and
tabs) are allowed between most items. Our task, which we choose to accept, is to
convert it to a regex as best we can. It will require every ounce of technique we
can muster, but it is possible. ***
Levels of interpretation
When building a regex using variables, you must take extra care to understand the
quoting, interpolating, and escaping that goes on. With ^\w+\@[.\w]+$ as
an example, you might naively render that as
$username = "\w+";
$hostname = "\w+(\.\w+)+";
$email = "^$username\@$hostname$";
... m/$email/o ...