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

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

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

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?

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.

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.