PHP regular expression $ {1} explains the solution

PHP regular expression $ {1} explains PHPcode & lt ;? Php $ stringaaaa3aaa6aaaa2aaaaaa0aaaaaa2aaa1a; $ patternarray (d) I); $ replacearray ([$ {1}]); echo $ string. & quot; & lt; br & gt; & quot; echopreg_repl PHP regular expression ${1}

PHP code

  

  "; Echo preg_replace ($ pattern, $ replace, $ string);?>

Before replacement: aaaa3aaa6aaaa2aaaaaa0aaaaaa2aaa1a
After replacement: aaaa [3] aaa [6] aaaa [2] aaaaaa [0] aaaaaaaa [2] aaa [1]
How can this problem be solved? How to explain the $ {1?

------ Solution --------------------
It is the meaning of the first parenthesis in pattern.
------ Solution --------------------
The first sub-mode. That is, the content of the first group.