Regular Expressions 101

Explanation
Pro

Non-Capturing Group(?:(?<=^|[^\w])(?<!@)@)

Positive Lookbehind(?<=^|[^\w])

1st Alternative^

^start of string

2nd Alternative[^\w]

Negated Character Class[^\w]

\wany word character

Negative Lookbehind(?<!@)

@the literal @ (64₁₀ / 100₈ / 40₁₆)

Group 1([\w.]*[\w])

Character Class[\w.]*

*matches any number of characters from the set below:

\wany word character

.the literal . (46₁₀ / 56₈ / 2E₁₆)

Character Class[\w]

\wany word character

Positive Lookahead(?=[^\w@]|$)

1st Alternative[^\w@]

Negated Character Class[^\w@]

\wany word character

@the literal @ (64₁₀ / 100₈ / 40₁₆)

2nd Alternative$

$end of string, or before its final line terminator

Negative Lookahead(?!@)

@the literal @ (64₁₀ / 100₈ / 40₁₆)

Ready to run

25 tokens

(?:(?<=^|[^\w])(?<!@)@)([\w.]*[\w])(?=[^\w@]|$)(?!@)