10.27 Using Patterns to Match Email Addresses and URLs
10.27.1 Problem
You want to determine whether or not
a value looks like an email address or a URL.
10.27.2 Solution
Use a pattern, tuned to the level of strictness you want to enforce.
10.27.3 Discussion
The immediately preceding sections use patterns to identify classes
of values such as numbers and dates, which are fairly typical
applications for regular expressions. But pattern matching has such
widespread applicability that it's impossible to
list all the ways you can use it for data validation. To give some
idea of a few other types of values that pattern matching can be used
for, this section shows a few tests for email addresses and URLs.
To check values that are expected to be email addresses, the pattern
should require at least an @ character with
nonempty strings on either side:
/.@./
That's a pretty minimal test. It's
difficult to come up with a fully general pattern that covers all the
legal values and rejects all the illegal ones, but
it's easy to write a pattern that's
at least a little more restrictive. For example, in addition to being nonempty, the username
and the domain name should consist entirely of characters other than
@ characters or spaces:
/^[^@ ]+@[^@ ]+$/
You may also wish to require that the domain name part contain at
least two parts separated by a dot:
/^[^@ ]+@[^@ .]+\.[^@ .]+/
To look for URL values that begin with a protocol specifier of
http://, ftp://, or
mailto:, use an alternation that matches any of
them at the beginning of the string. These values contain slashes, so
it's easier to use a different character around the
pattern to avoid having to escape the slashes with backslashes:
m#^(http://|ftp://|mailto:)#i
The alternatives in the pattern are grouped within parentheses
because otherwise the ^ will anchor only the first
of them to the beginning of the string. The i
modifier follows the pattern because protocol specifiers in URLs are
not case sensitive. The pattern is otherwise fairly unrestrictive,
because it allows anything to follow the protocol specifier. I leave
it to you to add further restrictions as necessary.
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