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Category
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Expression
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Description
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Example (Before)
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Example (After)
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Basics
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.
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Any character except a newline
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'hello world'
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'h'
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Basics
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^
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Start of a string
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'hello world'
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'hello'
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Basics
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$
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End of a string
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'hello world'
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'world'
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Basics
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\
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Escape special characters
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'Cost: 100$'
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'Cost: 100\$'
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Character Classes
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[abc]
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Matches a, b, or c
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'cat', 'bat', 'hat'
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'c', 'b', 'h'
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Character Classes
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[^abc]
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Negation: Matches anything except a, b, or c
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'dog', 'bat', 'rat'
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'd', 'r'
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Character Classes
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[a-z]
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Matches any lowercase letter (a to z)
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'apple', 'banana', 'grape'
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'a', 'b', 'g'
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Character Classes
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[A-Z]
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Matches any uppercase letter (A to Z)
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'APPLE', 'BANANA', 'GRAPE'
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'A', 'B', 'G'
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Character Classes
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[0-9]
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Matches any digit (0 to 9)
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'123', '456'
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'1', '4'
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Character Classes
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[a-zA-Z0-9_]
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Matches any alphanumeric character
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'word1', 'word_2', 'word3'
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'w', 'w', 'w'
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Quantifiers
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*
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0 or more times (greedy)
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'aaa', ''
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'aaa'
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Quantifiers
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+
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1 or more times (greedy)
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'a', 'aa', 'aaa'
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'a', 'aa', 'aaa'
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Quantifiers
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?
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0 or 1 time (optional)
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'a', ''
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'a'
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Quantifiers
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{n}
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Exactly n times
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'aaa', 'a'
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'aaa'
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Quantifiers
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{n,}
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At least n times
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'aa', 'aaa', 'aaaa'
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'aa', 'aaa'
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Quantifiers
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{n,m}
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Between n and m times
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'aaa', 'aa'
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'aaa', 'aa'
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Grouping & Alternation
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(abc)
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Grouping: Matches abc and captures it
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'abc', 'xyz'
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'abc'
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Grouping & Alternation
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(?:abc)
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Non-capturing group
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'abc', 'xyz'
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'abc'
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Grouping & Alternation
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a|b
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Matches a or b
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'a', 'b'
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'a', 'b'
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Assertions
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(?=abc)
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Positive lookahead: Ensures abc follows but doesn’t consume it
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'abc123'
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'123'
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Assertions
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(?!abc)
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Negative lookahead: Ensures abc does NOT follow
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'xyz123'
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'xyz123'
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Assertions
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(?<=abc)
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Positive lookbehind: Ensures abc precedes
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'abc'
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'abc'
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Assertions
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(?
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Negative lookbehind: Ensures abc does NOT precede
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'xyz'
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'xyz'
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Anchors & Word Boundaries
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\b
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Word boundary
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' word '
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' '
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Anchors & Word Boundaries
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\B
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Not a word boundary
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'word'
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'w'
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Common Escapes
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\d
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Any digit [0-9]
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'123'
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'1'
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Common Escapes
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\D
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Any non-digit [^0-9]
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'a'
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'a'
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Common Escapes
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\w
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Any word character [a-zA-Z0-9_]
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'abc123'
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'a', '1'
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Common Escapes
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\W
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Any non-word character [^a-zA-Z0-9_]
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'@#$%'
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'@'
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Common Escapes
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\s
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Any whitespace (spaces, tabs, newlines)
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' space '
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' '
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Common Escapes
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\S
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Any non-whitespace
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'word'
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'word'
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Common Examples
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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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'123-45-6789'
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'123-45-6789'
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Common Examples
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\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Z|a-z]{2,7}\b
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Email validation
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'user@example.com'
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'user@example.com'
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Common Examples
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https?://\S+
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Matches a URL
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'http://example.com'
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'http://example.com'
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Common Examples
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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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'123-456-7890'
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'123-456-7890'
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