.       # any character except newline
\w\d\s  # word, digit, whitespace
\W\D\S  # not word, digit, whitespace
[abc]   # any of a, b, or c
[^abc]  # not a, b, or c
[a-g]   # character between a & g

^abc$  # start/end of the string
\b\B   # word, not-word boundary

\.\*\\  # escaped special characters
\t\n\r  # tab, linefeed, carriage return

(abc)   # capture group
\1      # backreference to group #1
(?:abc) # non-capturing group
(?=abc) # positive lookahead
(?!abc) # negative lookahead

a*a+a?    # 0 or more, 1 or more, 0 or 1
a{5}a{2,} # exactly five, two or more
a{1,3}    # between one & three
a+?a{2,}? # match as few as possible
ab|cd     # match ab or cd