Encode contents between Two Special Strings - php

Question

All I want is to Get contents between two strings like the following line:
$content = '81Lhello82R 81Lmy82R 81Lwife82R';
I wish to get all contents between 81L and 82R, then encode them to Base64 automatically by Preg_match I think, I've done some ways to do it but didn't get what was expected!
Base Form:
81Lhello82R 81Lmy82R 81Lwife82R
Output:
81LaGVsbG8=82R 81LbXk=82R 81Ld2lmZQ==82R

Answer 1

Hard rules:
$leftMask = '81L';
$rightMask = '82R';
$content = '81Lhello82R 81Lmy82R 81Lwife82R';
preg_match_all('#'.$leftMask.'(.*)'.$rightMask.'#U',$content, $out);
$output = [];
foreach($out[1] as $val){
$output[] = $leftMask.base64_encode($val).$rightMask;
}
$result = str_replace($out[0], $output, $content);
RegExp rules
$leftMask = '\d{2}L';
$rightMask = '\d{2}R';
$content = '81Lhello82R 81Lmy82R 81Lwife82R';
preg_match_all('#('.$leftMask.')(.*)('.$rightMask.')#U',$content, $out);;
$output = [];
foreach($out[2] as $key=>$val){
$output[] = $out[1][$key].base64_encode($val).$out[3][$key];
}
$result = str_replace($out[0], $output, $content);

Answer 2

This is a job for preg_replace_callback:
$content = '81Lhello82R 81Lmy82R 81Lwife82R';
$output = preg_replace_callback(
'/(?<=\b\d\dL)(.+?)(?=\d\dR)/',
function($matches) {
return base64_encode($matches[1]); // encode the word and return it
},
$content);
echo $output,"\n";
Where
(?<=\b\d\dL) is a positive lookbehind that makes sure we have 2 digits and the letter L before the word to encode
(?=\d\dR) is a positive lookahead that makes sure we have 2 digits and the letter R after the word to encode
(.+?) is the capture group that contains the word to encode
Output:
81LaGVsbG8=82R 81LbXk=82R 81Ld2lmZQ==82R