Regex

#META CHARACTERS
.           #matches any single char except newline character
*           #match previous char zero or more times. is greedy.

#examples:
ca*t        #will match ct, cat, caat, caaat, and so on
a[bcd]*b    #will match abcb in abcbd
+           #match one or more times the previous char
?           #matches previous char/set once or zero times
{m, n}      #matches previous set/char repeated at least m times and at most n times

#META CHARACTERS
\           #Used to escape the meta-chars, and use them literally or use special sequences

#examples:
\s          #matches any whitespace char. equivalent to [ \t\n\r\f\v]
\S          #matches any non-whitespace char
\w          #matches any alpha-numeric char
\A          #matches the beginning of a (multi-line) string. Not the same as ^ in multi-line strings
\Z          #matches the end of string
\b          #word boundary. a word is a sequence of alpha-numeric chars. end of word is a whitespace or non-alphanumeric char. \bfoo\b will match the word foo

#META CHARACTERS
\1          #back reference group 1
[]          #Specifies a set of characters to match. Meta-chars are not active inside '[]'

#examples:
[^5]        #will match all chars except 5
[^A-Z]      #will match all except A-Z
[\s,.]      #matches any whitespace, comma, or dot
^           #Matches the beginning of a line
$           #Matches the end of a line
|           #given two regular Expression A and B, A|B matches either A or B (or both?)
()          #used to group chars. can be nested