Regex Tutorial

a, X, 9, < -- ordinary characters just match themselves exactly. The meta-characters which do not match themselves because they have special meanings are: . ^ $ * + ? { [ ] \ | ( ) (details below)
. (a period) -- matches any single character except newline '\n'
\w -- (lowercase w) matches a "word" character: a letter or digit or underbar [a-zA-Z0-9_]. Note that although "word" is the mnemonic for this, it only matches a single word char, not a whole word. \W (upper case W) matches any non-word character.
\b -- boundary between word and non-word
\s -- (lowercase s) matches a single whitespace character -- space, newline, return, tab, form [ \n\r\t\f]. \S (upper case S) matches any non-whitespace character.
\t, \n, \r -- tab, newline, return
\d -- decimal digit [0-9] (some older regex utilities do not support but \d, but they all support \w and \s)
^ = start, $ = end -- match the start or end of the string
\ -- inhibit the "specialness" of a character. So, for example, use . to match a period or \ to match a slash. If you are unsure if a character has special meaning, such as '@', you can put a slash in front of it, \@, to make sure it is treated just as a character.