python_regex_reference.txt

PYTHON REGULAR EXPRESSION QUICK GUIDE
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ANCHORS
^        Start of line or string
$        End of line or string

BASIC TOKENS
.        Any character except newline
\s       Whitespace (space, tab, newline)
\S       Non-whitespace
\d       Digit (0–9)
\D       Not a digit
\w       Word character (letter, digit, underscore)
\W       Not a word character
\b       Word boundary
\B       Not a word boundary

QUANTIFIERS
*        0 or more times
*?       0 or more times (non-greedy)
+        1 or more times
+?       1 or more times (non-greedy)
{n}      Exactly n times
{n,}     At least n times
{n,m}    Between n and m times

CHARACTER SETS
[aeiou]     Any vowel
[^XYZ]      Any character except X, Y, or Z
[a-z0-9]    Character range (a to z, 0 to 9)

GROUPS AND CAPTURING
(abc)          Capture group
(?:abc)        Non-capturing group
(?P<name>abc)  Named capture group
\1             Backreference to first group
(a|b)          Alternation – match "a" or "b"

LOOKAROUNDS
(?=...)   Positive lookahead
(?!...)   Negative lookahead
(?<=...)  Positive lookbehind
(?<!...)  Negative lookbehind

GREEDY VS NON-GREEDY
.+       Greedy – matches as much as possible
.+?      Non-greedy – matches as little as possible

ESCAPES
\.        Match a literal dot
\*        Match a literal asterisk
\+        Match a literal plus
\?        Match a literal question mark
\\        Escape the backslash itself

IMPORTANT PYTHON NOTES
Use raw strings for regex: r"pattern"

Common functions:
re.search()    Find first match
re.findall()   Return all matches
re.match()     Match only at start of string
re.sub()       Replace text using regex
re.split()     Split text by regex
re.compile()   Store compiled pattern

REGEX FLAGS
re.IGNORECASE or re.I     Ignore case
re.MULTILINE or re.M      ^ and $ match every line
re.DOTALL or re.S         . matches newline
re.VERBOSE                Multiline readable regex with comments

COMMON MINI-PATTERNS
Email:        [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[A-Za-z]{2,}
Phone:        \d{10}
URL:          https?://\S+
Date:         \d{2}[/-]\d{2}[/-]\d{4}
Words only:   [A-Za-z]+

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END OF QUICK GUIDE
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