Lookaheads and Lookbehinds

Zero-width assertions that check for patterns without consuming characters.

Positive Lookahead (?=...)

\d+(?= dollars)

Matches numbers followed by " dollars" (but doesn't include " dollars"):

• 50 dollars → matches 50
• 50 euros → no match

Negative Lookahead (?!...)

\d+(?! dollars)

Matches numbers NOT followed by " dollars":

• 50 dollars → no match
• 50 euros → matches 50

Password Validation with Lookaheads

^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)(?=.*[@$!%*?&])[A-Za-z\d@$!%*?&]{8,}$

Requires:

• At least one lowercase letter
• At least one uppercase letter
• At least one digit
• At least one special character
• Minimum 8 characters

Positive Lookbehind (?<=...)

(?<=\$)\d+

Matches numbers preceded by $ (but doesn't include $):

• $50 → matches 50
• €50 → no match

Negative Lookbehind (?<!...)

(?<!\$)\d+

Matches numbers NOT preceded by $:

• $50 → no match
• €50 → matches 50
• 50 → matches 50

Combining Lookahead and Lookbehind

(?<=\$)\d+(?=\.\d{2})

Matches dollar amounts with exactly 2 decimal places:

• $50.00 → matches 50
• $50.0 → no match
• 50.00 → no match (missing $)

Multiple Lookaheads

^(?=.*[A-Z])(?=.*\d).{6,}$

Matches strings that:

• Have at least one uppercase letter
• Have at least one digit
• Are at least 6 characters long

Word Boundaries with Lookaheads

\b\w+(?=ing\b)

Matches word stems before "ing":

• running → matches runn
• jumping → matches jump

Email Username Before @

\w+(?=@)

Matches email username (everything before @):

• user@example.com → matches user

Extract Version Numbers

(?<=version )\d+\.\d+\.\d+

Matches version numbers after "version ":

• version 1.2.3 → matches 1.2.3
• v1.2.3 → no match

Complex Price Pattern

(?<=\$)\d{1,3}(,\d{3})*(\.\d{2})?(?=\s|$)

Matches formatted prices after $:

• $1,234.56 → matches 1,234.56
• $999 → matches 999
• $1,234.5 → no match (wrong decimal format)

Exclude Specific Words

\b(?!and\b|or\b|the\b)\w+\b

Matches words except "and", "or", "the":

• the cat → matches cat only
• and then → matches then only

Find Numbers Not in Parentheses

(?<!\()\b\d+\b(?!\))

Matches numbers not surrounded by parentheses:

• (100) and 200 → matches 200
• (100) → no match