how could I combine these regex rules?

Question

I'm detecting @replies in a Twitter stream with the following PHP code using regexes.

$text = preg_replace('!^@([A-Za-z0-9_]+)!', '<a href="http://twitter.com/$1" target="_blank">@$1</a>', $text);
$text = preg_replace('! @([A-Za-z0-9_]+)!', ' <a href="http://twitter.com/$1" target="_blank">@$1</a>', $text);

How can I best combine these two rules without false flagging [email protected] as a reply?

Answer 1

Here's how I'd do the combination

$text = preg_replace('!(^| )@([A-Za-z0-9_]+)!', '$1<a href="http://twitter.com/$2" target="_blank">@$2</a>', $text);

Answer 2

OK, on a second thought, not flagging whatever@email means that the previous element has to be a "non-word" item, because any other element that could be contained in a word could be signaled as an email, so it would lead:

!(^|\W)@([A-Za-z0-9_]+)!

but then you have to use $2 instead of $1.

Answer 3

I think you can use alternation,: so look for the beginning of a string or a space

'!(?:^|\s)@([A-Za-z0-9_]+)!'

Answer 4

preg_replace('%(?<!\S)@([A-Za-z0-9_]+)%', '<a href="http://twitter.com/$1" target="_blank">@$1</a>', $text);

(?<!\S) is loosely translated to "no preceding non-whitespace character". Sort of a double-negation, but also works at the start of the string/line.

This won't consume any preceding character, won't use any capturing group, and won't match strings such as "[email protected]", which is a valid e-mail address.

Tested:

Input = 'foo bar [email protected] bee @def goo@doo @woo'
Output = 'foo bar [email protected] bee <a href="http://twitter.com/def" target="_blank">@def</a> goo@doo <a href="http://twitter.com/woo" target="_blank">@woo</a>'

Answer 5

Hu, guys, don't push too far... Here it is :

!^\s*@([A-Za-z0-9_]+)!

Answer 6

$text = preg_replace('/(^|\W)@(\w+)/', '<a href="http://twitter.com/$2" target="_blank">@$2</a>', $text);

Answer 7

Since the ^ does not have to stand at the beginning of the RE, you can use grouping and | to combine those REs.

If you don't want re-insert the whitespace you captured, you have to use "positive lookbehind":

$text = preg_replace('/(?<=^|\s)@(\w+)/',
    '<a href="http://twitter.com/$1" target="_blank">@$1</a>', $text);

or "negative lookbehind":

$text = preg_replace('/(?<!\S)@(\w+)/',
    '<a href="http://twitter.com/$1" target="_blank">@$1</a>', $text);

...whichever you find easier to understand.