If the text in your source file is <"\\[">, then your string becomes <\[> and your regex looks for a literal <[>

If the text in your source file is <"\[">, then your string becomes <[> and your regex sees the start of a character class.

OR, you can use single quotes to avoid the interpolation:

If the text in your source file is <'\\['>, then your string becomes <\\[> and your regex looks for a literal <\[>

If the text in your source file is <'\['>, then your string becomes <\[> and your regex looks for a literal <[>

If you are typing it as a one-liner into the command prompt, then you'll need twice as many backslashes of course.