WHY

$str = '[abc]d[ef]g123';
$reg = '(?<=\[[^[\]]*\]|^)[^[\]]+';
[regex]::Replace($str, $reg, {param($m), $m.Groups[0].Value.ToUpper()});
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结果：[abc]D[ef]G123

如果需要考虑括弧嵌套的问题：

$str = '[abc[w8w]]de[fg]xy123';
$reg = '\[(?:[^[\]]*|\[(?<open>)|\](?<-open>))*(?(open)(?!))\]|[^[\]]+';
[regex]::Replace($str, $reg, {param($m), $s = $m.Groups[0].Value; if($s.IndexOf('[')+1){$s}else{$s.ToUpper()}});
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结果：[abc[w8w]]DE[fg]XY123