preg_replace（）函数问题？

Question

Look at this simple script please

$a = "test";
echo $b = preg_replace('/[^a](e)/',"<b>$1</b>",$a); // returns <b>e</b>st

I want to bold the "e" characters, when there is no "a" character before it.

Logically in $0 it must match "te", and in $1 - "e", but why it strips the first character in my example?

I can solve the task in another way, but i want to understand this behavior.

Thanks much

Answer 1

why it strips the first character in my example?

Because the match is two characters wide: the e, and the character before it (which is denoted by [^a]).

To change this, there are two ways. The easy is just to parenthesize your match:

echo $b = preg_replace('/([^a])(e)/',"$1<b>$2</b>",$a); // returns t<b>e</b>st

The second is to use a negative lookbehind:

echo $b = preg_replace('/(?<!a)(e)/',"<b>$1</b>",$a); // returns t<b>e</b>st