RegEx NOT condition explanation and examples 

Creating a "NOT" condition in regular expressions can be achieved primarily through two mechanisms: negated character classes and negative lookarounds.

1. Negated Character Classes

This method is used to match any single character except those specified within the class.

• Syntax: [^...]
• Explanation: The caret ^ at the beginning of a character class [] negates the class, meaning it will match any character that is not present within the brackets.
• Example: [^aeiou] matches any single character that is not a lowercase vowel.

2. Negative Lookarounds

Negative lookarounds are zero-width assertions that check for the absence of a pattern without consuming any characters in the string.

a) Negative Lookahead

• Syntax: (?!pattern)
• Explanation: This asserts that pattern does not immediately follow the current position.
• Example: ^(?!word).*$ matches an entire line that does not contain the substring "word".
  • ^: Matches the beginning of the line.
  • (?!word): Negative lookahead ensures "word" is not present immediately after the start of the line.
  • .*: Matches any character (except newline) zero or more times.
  • $: Matches the end of the line.

b) Negative Lookbehind

• Syntax: (?<!pattern)
• Explanation: This asserts that pattern does not immediately precede the current position.
• Example: .{3}(?<!foo|bar) matches any three-character string that is not preceded by "foo" or "bar".

Choosing the Right Method

• Negated character classes: are suitable for negating individual characters or sets of characters.
• Negative lookarounds: are necessary when you need to assert the absence of a specific pattern (a sequence of characters or a more complex regex) at a particular position without consuming those characters.