Positive Lookahead
Assert that pattern is followed by another.
(?=...) # Positive lookahead
\d(?=px) # Digit followed by "px"
\w+(?=@) # Word before @
test(?=ing) # "test" followed by "ing"Negative Lookahead
Assert that pattern is NOT followed by another.
(?!...) # Negative lookahead
\d(?!px) # Digit NOT followed by "px"
\w+(?!@) # Word NOT before @
test(?!ing) # "test" NOT followed by "ing"Positive Lookbehind
Assert that pattern is preceded by another.
(?<=...) # Positive lookbehind
(?<=\$)\d+ # Digits after $
(?<=@)\w+ # Word after @
(?<=Mr\.)\w+ # Word after "Mr."Negative Lookbehind
Assert that pattern is NOT preceded by another.
(?<!...) # Negative lookbehind
(?<!\$)\d+ # Digits NOT after $
(?<!@)\w+ # Word NOT after @
(?<!un)\w+ # Word NOT after "un"Combining Lookarounds
Use multiple assertions together.
(?=.*[a-z])(?=.*[A-Z])(?=.*\d).{8,}
# Password: lowercase, uppercase, digit, 8+ chars
(?<=<)(?!/)(\w+)(?=>)
# HTML opening tag name
\b(?!cat|dog)\w+\b
# Word not "cat" or "dog"Practical Examples
Real-world lookaround usage.
(?=.*\d)(?=.*[a-z])(?=.*[A-Z]).{6,}
# Strong password validation
(?<=\$)\d+\.?\d*
# Dollar amount without $
\b\w+(?=\.com)
# Domain name before .com
(?<!\/\/).*(?=:)
# Protocol in URL
(?!.*test).*
# Line not containing "test"Password Validation
Complex password requirements.
(?=.*[a-z]) # At least one lowercase
(?=.*[A-Z]) # At least one uppercase
(?=.*\d) # At least one digit
(?=.*[@$!%*?&]) # At least one special char
.{8,} # At least 8 characters
Combined:
^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$Common mistakes / Pitfalls
- People often copy a command or pattern without adapting placeholders, which can break production workflows unexpectedly.
- It is easy to forget environment-specific differences, so always verify behavior in your shell, runtime, or API gateway before shipping.
- Many errors come from skipping small validation steps, so test with realistic sample input before relying on the result.
Last updated: February 2026