How does "positive" (?=X) work combined with "positive" lookbehind (?<=X)?

Question

After answering this question, and despite can understand what is happening in the regular expression of my reply, I was curious to know how the excerpt ((?<=;)|(?=;)) works.

I read in this reply what each one does, and also in various other sources, but I confess that I did not understand well how the expression cited works.

If possible, I’d like an explanation, because I’m trying to learn regular expression but the explanations out there are complicated and with terms that I’m often not familiar with.

Answer 1

You must have heard that phrase that says :

Just because you’ve reached your goal doesn’t mean you’re right

Well, what happened with your REGEX is the following, I will explain with another to be clearer.

/((?<=t)|(?=a)).+/

Explanation

• ((?<=t)|(?=a)) - Group in which one of the occurrences must occur, giving preference to the first (?<=t)
• .+ - Anything you can get and having at least one.

So we could dismember into two REGEX :

• /(?<=t).+/ - Anything that comes after t
• /(?=a).+/ - Anything you got a

Testing

$regex = '~((?<=t)|(?=a)).+~';

$testes = array(
    'ana',
    'tania',
    'anastasia',
    'etilico',
    'aguilherme'
    'guilherme'
);

foreach ($testes as $k => $value){
    preg_match($regex, $value, $matches);
    print_r($matches);
}

Exit

[0] => ana
[0] => ania
[0] => anastasia
[0] => ilico
[0] => aguilherme
[0] => 

Taking up your REGEX ((?<=;)|(?=;))

It’s redundant because they’re both checking ; so if there is of the sentence (?=;) then it will also occur (?<=;).
Yet there is one thing, (?<=...) depends on what comes next, in such a way that if it is .+, but the comma come at the end of the sentence ;$, the second part will not be completed .+, so falling in the second (?=;).

Doubts

This last explanation can leave a little confused, any doubt ask.

Addendum - as to your doubt

The problem I had was that when I used Lookahead, the comma was not isolated and broken into an inlet by the java split when it was preceded by another string, and the inverse occurred when it was the lookbehind see so the confusion to understand each.

What happens is the following :

Both REGEX are "exploding" by merging characters (this example group 2), but that (?<=;) is more specific than (?=;), reminding above:

• (?<=;) - Whatever comes after ;
• (?=;) - sentence that contains ;

Thus (?<=;) will explode by the junction that comes after the ; forming the words you saw pontoevirgula;, delinha;.

But the (?=;) could explode with both what comes before and what will come after, however the split consumes the character after using it, this way only occurs the explosion by the first (that comes before) generating the other words you saw: ;espaco, ;QUEBRA