What regular expression can never match?

There is no single regular expression that can never match. However, there are some regular expressions that are designed not to match anything. For example, the regular expression (a|b)*a^ will not match any string because the ^ character matches the start of a line, but the * quantifier allows for zero or more matches of the preceding pattern (a|b), which means that the match can be empty. As a result, the ^ character is not matched, and the expression does not match anything.

Another example of a regular expression that will not match anything is (?!.*), which is a negative lookahead assertion that fails if any characters are present in the input string.

It’s important to note that the ability to match or not match a string depends on the implementation and syntax of the regular expression engine you are using. Different engines may have different features and syntax variations, so it’s important to consult the documentation for the specific engine you are using to determine what expressions are supported and how they behave.

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