与模式匹配或为空字符串的正则表达式

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, eg 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, ie 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):
  • ie between a word character and a non-word character
• Between ^ and \\w
  • ie at the beginning of the string if it starts with \\w
• Between \\w and $
  • ie 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 (ie 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.

Answer 5

^$ did not work for me if there were multiple patterns in regex.

Another solution:

/(pattern1)(pattern2)?/g

"pattern2" is optional. If empty, not matched.

? matches (pattern2) between zero and one times.

Tested here ("m" is there for multi-line example purposes): https://regex101.com/r/mezfvx/1