Regular expression replace whole words in PHP

Question

I have an expression like:

$x="We have a cat here.";

I want to replace "a" with, for example 5, so this should look like:

We have 5 cat here. //It doesn't make sense, but it's just an example.

I tried simple

echo str_replace("a","5",$x);

but that returned

We h5ve 5 c5t here.

Then, I tried

echo str_replace(" a "," 5 ",$x);

but that didn't do the job for strings like

We have a cat here. (A dog actually).

I decided to use regex, but Im completely newbie to this, and I don't know how to use any of them... Well, I'd really appreciate any link to a good tutorial, but I need the answer quite fast...

Answer 1

使用\\b标记单词边界，例如

$newstring = preg_replace('/\b[Aa]\b/', '5', $string);