Hai cercato la traduzione di backreference da Coreano a Inglese

character with octal code ddd, or backreference
if a pattern starts with .* or .{ 0,} and the pcre_dotall option (equivalent to perl's / s) is set, thus allowing the. to match newlines, then the pattern is implicitly anchored, because whatever follows will be tried against every character position in the subject string, so there is no point in retrying the overall match at any position after the first.

참고: when working with a replacement pattern where a backreference is immediately followed by another number (i.e.: placing a literal number immediately after a matched pattern), you cannot use the familiar \\1 notation for your backreference. \\11, for example, would confuse preg_replace() since it does not know whether you want the \\1 backreference followed by a literal 1, or the \\11 backreference followed by nothing. in this case the solution is to use \${1}1. this creates an isolated $1 backreference, leaving the 1 as a literal.
in this case the solution is to use \${ 1 }1.