Update a regex that matches twitter like mentions to allow for dots

Question

I have already found helpful answers for a regex that matches twitter like username mentions in this answer and this answer

(?<=^|(?<=[^a-zA-Z0-9-_\.]))@([A-Za-z]+[A-Za-z0-9_]+)

(?<=^|(?<=[^a-zA-Z0-9-_\.]))@([A-Za-z]+[A-Za-z0-9-_]+)

However, I need to update this regex to also include usernames that has dots.

• One or more dots are allowed in a username.
• The username must not start or end with a dot.
• No two consecutive dots are allowed.

Example of a matched string:

@valid.user.name
^^^^^^^^^^^^^^^^

Examples of non-matched strings:

@.user.name // starts with a dot
@user.name. // ends with a dot
@user..name // has two consecutive dots

Answer 1

You can use this refactored regex:

(?<=[^\w.-]|^)@([A-Za-z]+(?:\.\w+)*)$

RegEx Demo

RegEx Details:

• (?<=[^\w.-]|^): Lookbehind to assert that we have start of line or any non-word, non-dot, non-hyphen character before current position
• @: Match literal `@1
• (: Start capture group
  • [A-Za-z]+: Match 1+ ASCII letters
  • (?:\.\w+)*: Match 0 or more instances of dot followed 1+ word characters
• ): End capture group
• $: End

Answer 2

The (?<=^|(?<=[^a-zA-Z0-9-_\.])) is a positive lookbehind that requires a match to be at the start of the string or right after an alphanumeric, -, _, ., you may write it in a more compact way as (?<![\w.-]), a negative lookbehind.

Next, ([A-Za-z]+[A-Za-z0-9_]+) captures 1+ ASCII letters and then 1+ ASCII letters or/and underscores. You seem to make sure the first char is a letter, then any number of sequences of . and 1+ word chars are allowed, that is, you may use [A-Za-z]\w*(?:\.\w+)*.

As you do not want to match it if there is a . right after the expected match, you need to set a lookahead that will require a space or end of string, (?!\S).

So, combining it, you can use

'~(?<![\w.-])@([A-Za-z]\w*(?:\.\w+)*)(?!\S)~'

See the regex demo

Details

• (?<![\w.-]) - no letters, digits, _, . and - immediately to the left of the current location are allowed
• @ - a @ char
• ([A-Za-z]\w*(?:\.\w+)*) - Group 1:
  • [A-Za-z] - an ASCII letter
  • \w* - 0+ letters, digits, _
  • (?:\.\w+)* - 0+ sequences of
    • \. - dot
    • \w+ - 1+ letters, digits, _
• (?!\S) - whitespace or end of string are required immediately to the right of the current location.

Answer 3

EDIT: Simpler version (same result)

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

Original

Another one:

^@[a-zA-Z][a-zA-Z_-]?(\.?[\w\d-]+){0,}$

• ^@ starts with @
• [a-zA-Z] first char
• [a-zA-Z_-]? match a-zA-Z_- 0 or more times
• ( start group
  • \.? match . (optional)
  • [\w\d-]+ match a-zA-Z0-9-_ 1 or more times
• ) end group
• {0,} repeat group 0 to infinite times
• $ end

Tests

valid:

@validusername
@valid.user.name
@valid-user-name
@valid_user-name
@valid-user123_name
@a.valid-user123_name

not valid:

@-invalid.user
@_invalid.user
@1notvalid-user_123name33
@.user.name
@user.name.
@user..name