PHP converting plain text to hashtag link

You could modify your regex to include a negative lookbehind for a non-whitespace character, like so:

(?<!\S)#([0-9a-zA-Z]+)

Working regex example:

http://regex101.com/r/mR4jZ7

PHP:

$text = preg_replace('/(?<!\S)#([0-9a-zA-Z]+)/', '<a href="/hashtag/$1">#$1</a>', $text);

Edit: And to make the expression compatible with other languages (non-english characters):

(?<!\S)#([0-9\p{L}]+)

Working example:

https://regex101.com/r/Pquem3/1

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