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^         Match the start of a string
$         Match the end of a string
\b        Match a word boundary
\B        Match a non-word boundary

^Hello    Matches "Hello world" but not "Say Hello"
world$    Matches "Hello world" but not "world peace"
\bcat\b   Matches "cat" in "the cat sat" but not in "category"
\Bcat     Matches "cat" in "category" but not in "the cat"
\bcat     Matches "cat" at word start: "cat" and "category"
cat\b     Matches "cat" at word end: "cat" and "tomcat"

[xyz]     Match any of x, y or z
[^xyz]    Match anything except x, y or z
[0-9]     Match any digit
[a-z]     Match any lowercase letter

[aeiou]   Matches any vowel
[^aeiou]  Matches any consonant (non-vowel)
[0-9]     Matches "5" in "Hello5"
[a-zA-Z]  Matches any letter (upper or lower)
[a-z0-9]  Matches any lowercase letter or digit
gr[ae]y   Matches both "gray" and "grey"
[^0-9]    Matches any non-digit character

\.        Matches a literal period/dot character
\d        Matches any digit (0-9)
\s        Matches any whitespace (space, tab, newline)
\w        Matches any word character (a-z, A-Z, 0-9, _)
\D        Matches any non-digit
\S        Matches any non-whitespace
\W        Matches any non-word character

192\.168\.1\.1   Matches IP address (dots are literal)
\d{3}            Matches "123" in "Room 123"
Hello\sWorld     Matches "Hello World" or "Hello World"
\w+              Matches "hello", "test123", "my_var"
\d+\.\d+         Matches decimal numbers like "3.14" or "99.99"
\w+@\w+\.\w+     Matches simple email pattern

i         Case-insensitive matching
g         Global match (find all matches)
m         Multiline mode (^ and $ match line breaks)
s         Dotall mode (. matches newline)

*         Greedy: match as much as possible
*?        Lazy: match as little as possible
+         Greedy
+?        Lazy
?         Greedy
??        Lazy
{n,m}     Greedy
{n,m}?    Lazy

(xyz)     Match 'xyz' as group
(x|y)     Match 'x' or 'y'
(?=xyz)   Match succeeds only if followed by xyz
(?!xyz)   Match succeeds only if not followed by xyz

(cat|dog)        Matches "cat" or "dog"
(\d{3})-(\d{4})  Matches "555-1234" (captures area code and number separately)
(https?://)?     Matches optional "http://" or "https://"
test(?=ing)      Matches "test" in "testing" but not in "tester"
test(?!ing)      Matches "test" in "tester" but not in "testing"
\d+(?=px)        Matches "10" and "20" in "10px 15em 20px"
\$\d+(?!\.)      Matches "$50" but not "$50.99" (whole dollars only)

\1        Reference to first captured group
\2        Reference to second captured group

\t        Tab character
\n        Newline
\r        Carriage return
\\        Literal backslash
\*        Literal asterisk
\+        Literal plus
\?        Literal question mark

\t           Matches tab in "Hello\tWorld"
\n           Matches newline in multi-line text
Price: \$50  Matches "Price: $50" (literal dollar sign)
C:\\Users    Matches "C:\Users" (literal backslash)

?         Match 0 or 1 time (optional)
*         Match 0 or more times
+         Match 1 or more times (at least once)
{n}       Match exactly n times
{n,}      Match n or more times
{n,m}     Match between n and m times

a?        Matches "" or "a"
a*        Matches "", "a", "aa", "aaa", etc.
a+        Matches "a", "aa", "aaa", etc. (but not empty)
a{3}      Matches exactly "aaa"
a{2,}     Matches "aa", "aaa", "aaaa", etc.
a{2,4}    Matches "aa", "aaa", or "aaaa"