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1. Basics
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Expression
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Description
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.
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Any character except a newline
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^
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Start of a string
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$
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End of a string
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\
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Escape special characters (e.g., \. matches a literal dot)
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2. Character Classes
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Expression
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Description
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[abc]
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Matches a, b, or c
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[^abc]
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Negation: Matches anything except a, b, or c
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[a-z]
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Matches any lowercase letter (a to z)
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[A-Z]
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Matches any uppercase letter (A to Z)
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[0-9]
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Matches any digit (0 to 9)
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[a-zA-Z0-9_]
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Matches any alphanumeric character
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3. Quantifiers
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Expression
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Description
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*
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0 or more times (greedy)
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+
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1 or more times (greedy)
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?
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0 or 1 time (optional)
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{n}
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Exactly n times
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{n,}
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At least n times
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{n,m}
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Between n and m times
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4. Grouping & Alternation
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Expression
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Description
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(abc)
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Grouping: Matches abc and captures it
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(?:abc)
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Non-capturing group
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`a
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b`
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5. Assertions (Lookaheads & Lookbehinds)
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Expression
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Description
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(?=abc)
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Positive lookahead: Ensures abc follows but doesn’t consume it
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(?!abc)
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Negative lookahead: Ensures abc does NOT follow
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(?<=abc)
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Positive lookbehind: Ensures abc precedes
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(?<!abc)
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Negative lookbehind: Ensures abc does NOT precede
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6. Anchors & Word Boundaries
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Expression
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Description
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\b
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Word boundary
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\B
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Not a word boundary
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7. Common Escapes
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Expression
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Description
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\d
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Any digit [0-9]
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\D
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Any non-digit [^0-9]
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\w
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Any word character [a-zA-Z0-9_]
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\W
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Any non-word character [^a-zA-Z0-9_]
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\s
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Any whitespace (spaces, tabs, newlines)
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\S
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Any non-whitespace
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8. Common Examples
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Pattern
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Matches
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^\d{3}-\d{2}-\d{4}$
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Matches SSN format (e.g., 123-45-6789)
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`\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+.[A-Z
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a-z]{2,7}\b`
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https?://\S+
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Matches a URL
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\b\d{3}-\d{3}-\d{4}\b
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Matches phone number (123-456-7890)
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