<?php
$text = "a b c d [a b c Str1 d e f Str2 g h i Str3 ] e f g Str4 [Str5 a b c Str6 d] h i";
$re1 = "/\\[[^\\]]*\\]/s";
$re2 = "/\\bStr\\d+\\b/";
preg_match_all( $re1, $text, $arr1 );
foreach ( $arr1[0] as $k=>$v ) {
	preg_match_all( $re2, $v, $arr2 );
	// обработка результата, например так:
	var_dump( $arr2[0] );
};