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?