JavaScript Why does lookahead check every character?

Yes, it does check every character that the [\x00-\x7F] matches.

The negative lookahead here is used to restrict the [\x00-\x7F] character class. The pattern can be described as match a whole string that consists of only ASCII characters excluding ' and ". Since you applied a * quantifier to ((?!['"])[\x00-\x7F]) group, the regex engine tries to match this pattern zero or more times, from the start of the string to its end, thus, checking each position inside it. You could make this expression more efficient if you split the range as:

^[\x00-\x21\x23-\x26\x28-\x7F]*$

You would avoid the backtracking caused by using a lookahead (note that the " is \x22 and ' is \x27).

See the regex demo:

enter image description here

Answer:1

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