how to read html code characters in @Html.TextArea?

Use Regex to replace [b] with <b> and so on, google for regex replace examples and you'll probably find something relevant. You could use a standard string.Replace but you have to deal with the case where there are opening tags but no closing tags, so you really want to replace

[b]***anthing***[/b]

with

<b>****anything</b>

such that this

[b]***anthing***

remains as it is untouched, and you really need regex for that.