How can individual characters be matched in a given sequence, with the exception of specific expressions?

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One way to match individual characters in a given sequence, with the exception of specific expressions is by using negative lookahead and lookbehind assertions in regular expressions.

Negative lookahead assertion is denoted by (?!expression) and it matches any character that is not immediately followed by the expression.

Negative lookbehind assertion is denoted by (?<!expression) and it matches any character that is not immediately preceded by the expression.

For example, if we want to match all occurrences of the character 'a' in a given sequence, except when it is followed by the character 'b', we can use the following regular expression:

a(?!b)

This regex matches 'a' only if it is not followed by 'b'.

Similarly, if we want to match all occurrences of the character 'a' in a given sequence, except when it is preceded by the character 'b', we can use the following regular expression:

(?<!b)a

This regex matches 'a' only if it is not preceded by 'b'.

These negative assertions can be combined with other regular expression patterns to create more complex matching rules.