Php / String / Regex - https://www.minte9.com/php/string-regexp-71

index.php

<?php

/**
 * Replace
 * {b} abc {/b} with:
 * <b> abc </b>
 * Use preg_replace() function
 */
$str = "{b} abc {/b}";

/**
 * SOLUTION
 */
$pattern = "/{b} (.*) {\/b}/";
preg_match($pattern, $str, $matches);
print_r($matches); // [1] => "abc"

$replace = preg_replace($pattern, "<b> $1 </b>", $str);
echo $replace; // <b> abc </b>