I have a rich text like

Sample text for testing:<a href="http://www.baidu.com" title="leoshi">leoshi</a>leoshi for details balala...
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What I want to match is the word leoshi, but not inside of <a> elements, So in this example it's only leoshi in leoshi for details....

Solution and explanation are welcome!

A trick aimed to handle such "find a word but not a specific context" cases is described here: http://www.rexegg.com/regex-best-trick.html.

In essence it is: match your word in the undesired context or (using alternation) just this word but in a capture group. Then analyze the captures.

The regex in your case would be:

<a.*?>.*leoshi.*<\/a>|(leoshi)

Demo: https://regex101.com/r/zO0tV2/1

Then you need to check captures:

var input = "...";
var pattern = /<a.*?>.*leoshi.*<\/a>|(leoshi)/;
var match = pattern.exec(input);
var inputMatches = match !== null && match[1] !== null;

Demo: https://ideone.com/KkAl2I