<?php
$string = 'I want to be transformed to a proper link: http://www.google.com But please leave me alone (<a href="https://www.google.com">https://www.google.com</a>).';
$regex = '~            # delimiter
              (?<![">])    # a neg. lookbehind
              https?://\S+ # http:// or https:// followed by not a whitespace
              \b           # a word boundary
              ~x';         # verbose to enable this explanation.
$string = preg_replace($regex, "<a href='$0'>$0</a>", $string);
echo $string;
?>

PD9waHAKJHN0cmluZyA9ICdJIHdhbnQgdG8gYmUgdHJhbnNmb3JtZWQgdG8gYSBwcm9wZXIgbGluazogaHR0cDovL3d3dy5nb29nbGUuY29tIEJ1dCBwbGVhc2UgbGVhdmUgbWUgYWxvbmUgKDxhIGhyZWY9Imh0dHBzOi8vd3d3Lmdvb2dsZS5jb20iPmh0dHBzOi8vd3d3Lmdvb2dsZS5jb208L2E+KS4nOwokcmVnZXggPSAnfiAgICAgICAgICAgICMgZGVsaW1pdGVyCiAgICAgICAgICAgICAgKD88IVsiPl0pICAgICMgYSBuZWcuIGxvb2tiZWhpbmQKICAgICAgICAgICAgICBodHRwcz86Ly9cUysgIyBodHRwOi8vIG9yIGh0dHBzOi8vIGZvbGxvd2VkIGJ5IG5vdCBhIHdoaXRlc3BhY2UKICAgICAgICAgICAgICBcYiAgICAgICAgICAgIyBhIHdvcmQgYm91bmRhcnkKICAgICAgICAgICAgICB+eCc7ICAgICAgICAgIyB2ZXJib3NlIHRvIGVuYWJsZSB0aGlzIGV4cGxhbmF0aW9uLgokc3RyaW5nID0gcHJlZ19yZXBsYWNlKCRyZWdleCwgIjxhIGhyZWY9JyQwJz4kMDwvYT4iLCAkc3RyaW5nKTsKZWNobyAkc3RyaW5nOwo/Pg==