How do you find out if a user has entered a valid email address? Do you check for an at-sign, or is it more complicated? For many developers the answer is a regular expression, a little bit of code that can describe text patterns using wildcards and other special characters. If you’re new to the topic, we have a great regular expression tutorial.

Here are four regular expressions (often called regexes) that all validate the format of an email address. They have increasing degrees of complexity. The more complicated, the more accurate each is at matching only email addresses.

1. Dirt-simple approach

Here’s a regex that only requires a very basic xxxx@yyyy.zzz:

.+\@.+\..+

Upside: Dirt simple.

Downside: Even invalid email addresses like xxxx@yyyy.zzz, or even a@b.c, make it through.

2. Slightly more strict (but still simple) approach

Regular-Expressions.Info provides a basic email validation regex that tries to be a little smarter:

[A-Z0-9._%+-]+@[A-Z0-9.-]+\.[A-Z]{2,4}

Upside: Only allows email address-friendly characters, restricts domain extension to only two to four characters.

Downside: It still allows many invalid email addresses, and misses some longer domain extensions (.museum, for example).

3. Specify all the domain extensions approach

Reddit user teye points to his regex, which only allows domain extensions that actually exist:

([a-z0-9][-a-z0-9_\+\.]*[a-z0-9])@([a-z0-9][-a-z0-9\.]*[a-z0-9]\.(arpa|root|aero|biz|cat|com|coop|edu|gov|info|int|jobs|mil|mobi|museum|name|net|org|pro|tel|travel|ac|ad|ae|af|ag|ai|al|am|an|ao|aq|ar|as|at|au|aw|ax|az|ba|bb|bd|be|bf|bg|bh|bi|bj|bm|bn|bo|br|bs|bt|bv|bw|by|bz|ca|cc|cd|cf|cg|ch|ci|ck|cl|cm|cn|co|cr|cu|cv|cx|cy|cz|de|dj|dk|dm|do|dz|ec|ee|eg|er|es|et|eu|fi|fj|fk|fm|fo|fr|ga|gb|gd|ge|gf|gg|gh|gi|gl|gm|gn|gp|gq|gr|gs|gt|gu|gw|gy|hk|hm|hn|hr|ht|hu|id|ie|il|im|in|io|iq|ir|is|it|je|jm|jo|jp|ke|kg|kh|ki|km|kn|kr|kw|ky|kz|la|lb|lc|li|lk|lr|ls|lt|lu|lv|ly|ma|mc|md|mg|mh|mk|ml|mm|mn|mo|mp|mq|mr|ms|mt|mu|mv|mw|mx|my|mz|na|nc|ne|nf|ng|ni|nl|no|np|nr|nu|nz|om|pa|pe|pf|pg|ph|pk|pl|pm|pn|pr|ps|pt|pw|py|qa|re|ro|ru|rw|sa|sb|sc|sd|se|sg|sh|si|sj|sk|sl|sm|sn|so|sr|st|su|sv|sy|sz|tc|td|tf|tg|th|tj|tk|tl|tm|tn|to|tp|tr|tt|tv|tw|tz|ua|ug|uk|um|us|uy|uz|va|vc|ve|vg|vi|vn|vu|wf|ws|ye|yt|yu|za|zm|zw)|([0-9]{1,3}\.{3}[0-9]{1,3}))

Upside: It doesn’t allow xxxx@yyyy.zzz!

Downside: Upkeep could be tough with this one. You’d have to update any time new domain extensions are announced. In fact, you already would need to add the .me extension.

4. Way complicated approach

A Perl module has a long regular expression based on the standard description of an email address. It’s so long (nearly 6,500 characters!) that I won’t include it here.

Upside: It’s complete.

Downside: It’s way complicated.

Meet in the middle approach

You’ll have to decide, if you haven’t already, which regular expression to use. Likely, you’ll choose somewhere in the middle of the examples we’ve given. Regular-Expressions.Info has a good run-down of the trade-offs of different approaches.

Have you already decided how to check email addresses? How do you do it?

[via Reddit]

See also:

• A New Tool Offers a Little Help with Regular Expressions
• June 1 Is Regular Expression Day
• Tutorial: Use Regex in Perl