RegExp doubt regarding symbols

Trying to understand and replicate a text highlighter in JS and came across this particular command:

textToSearch = textToSearch.replace(/[.*+?^${}()|[\]\\]/g,"\\$&"); 

as shown in the photo below. What does the set of symbols followed by a "/g " mean ? What does β€œ\$&” mean ???

HTML code
HTML code

JS code

that’s a regular expression with a g (global) flag, the flags go after the closing / of the regular expression

this is replace syntax, let’s look at the documentation:

$& will add the matched string in the string to substitute

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Thanks for the help!!!