正则表达式匹配后，BASH_REMATCH显示空字符串

Question

只是想学一下BASH_REMATCH。

在ubuntu16.10上，我为BASH_REMATCH写了一个小脚本：

#!/bin/bash
x='sadas123fnrejkwhfklde'
if [[ $x =~ ([0-9]*) ]] 
then
  echo "match"
  echo ${BASH_REMATCH[1]}
else
  echo "not match"
fi

然而，当/i运行此脚本时，只显示“匹配”，而不显示捕获的匹配！

bash版本为：

$ bash --version
GNU bash, version 4.3.48(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

任何反馈都是值得感谢的。