在查找电子邮件地址时，如何使PHP正则表达式断言适用于整个模式？

Question

I am trying to find email addresses in text posted by users to my online forum, and change them into Bulletin Board Code (BBCode). So for instance:

[e-mail] 

would become:

[mail][e-mail][/mail]  

However I don't want to change email addresses that are already in BBCode. So for instance, I don't want:

[mail][e-mail][/mail]

to become:

[mail][mail][e-mail][/mail][/mail]

Therefore I need to add a negative lookbehind assertion to my regular expression to ensure that an email address is not preceeded by the characters [mail] (or just ]).

The PHP code I'm using is:

$pattern = '#(?<!])([a-zA-Z0-9_\-\.]*@\S+\.\w+)#';
$bbcode = '[mail]$1[/mail]';
preg_replace($pattern, $bbcode, $text);

The problem I have is that the negative lookbehind is only applied to the first character for the email address seeking sub-pattern. For example when applied to the text:

[mail][e-mail][/mail]

The result is:

[mail]e[mail][e-mail][/mail][/mail]

So the negative lookbehind finds [mail]e, but the rest of the email address [e-mail] still validates. I reaise that this is because of the way the email address seeking sub-pattern is written, because it allows for any number of characters before the @.

How can I change the regular expression to make the negative lookbehind appliy to the whole of the email address seeking sub-pattern, whilst still successfully catching the majority of email addresses being posted?

Answer 1

You need to have a word boundary at the beginning of regex to avoid matching the text partially, and also use + instead of * for the username part in email regex. Try using this regex,

(?<!])\b([a-zA-Z0-9_\-\.]+@\S+\.\w+)(?!\[)

Demo