Regular expression which matches a pattern, or is an empty string

Question

I have the following Regular Expression which matches an email address format:

^[\w\.\-]+@([\w\-]+\.)+[a-zA-Z]+$

This is used for validation with a form using JavaScript. However, this is an optional field. Therefore how can I change this regex to match an email address format, or an empty string?

From my limited regex knowledge, I think \b matches an empty string, and | means "Or", so I tried to do the following, but it didn't work:

^[\w\.\-]+@([\w\-]+\.)+[a-zA-Z]+$|\b

Answer 1

To match pattern or an empty string, use

^$|pattern

Explanation

• ^ and $ are the beginning and end of the string anchors respectively.
• | is used to denote alternates, e.g. this|that.

References

• regular-expressions.info/Anchors and Alternation

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On \b

\b in most flavor is a "word boundary" anchor. It is a zero-width match, i.e. an empty string, but it only matches those strings at very specific places, namely at the boundaries of a word.

That is, \b is located:

• Between consecutive \w and \W (either order):
  • i.e. between a word character and a non-word character
• Between ^ and \w
  • i.e. at the beginning of the string if it starts with \w
• Between \w and $
  • i.e. at the end of the string if it ends with \w

References

• regular-expressions.info/Word Boundaries

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On using regex to match e-mail addresses

This is not trivial depending on specification.

Related questions

• What is the best regular expression for validating email addresses?
• Regexp recognition of email address hard?
• How far should one take e-mail address validation?

Answer 2

An alternative would be to place your regexp in non-capturing parentheses. Then make that expression optional using the ? qualifier, which will look for 0 (i.e. empty string) or 1 instances of the non-captured group.

For example:

/(?: some regexp )?/

In your case the regular expression would look something like this:

/^(?:[\w\.\-]+@([\w\-]+\.)+[a-zA-Z]+)?$/

No | "or" operator necessary!

Here is the Mozilla documentation for JavaScript Regular Expression syntax.

Answer 3

I'm not sure why you'd want to validate an optional email address, but I'd suggest you use

^$|^[^@\s]+@[^@\s]+$

meaning

^$        empty string
|         or
^         beginning of string
[^@\s]+   any character but @ or whitespace
@         
[^@\s]+
$         end of string

You won't stop fake emails anyway, and this way you won't stop valid addresses.

Answer 4

\b matches a word boundary. I think you can use ^$ for empty string.