character sets:
	[chars]
	negated:  [^chars]
	shorthands:
		word characters: \w [A-Za-z0-9_]
		whitespace: \s [ \t\r\n]
		digit: \d [0-9]
		negated: upper

delimiters:
	start of line: ^
	end of line: $
		with preg_replace use /m
	start of string: \A
	end of string: \Z
	start/end of word: \b

php
	pattern modifiers:
		http://php.net/manual/en/reference.pcre.pattern.modifiers.php
			m: multiline (to match line-start with ^ and line-end with $)
		insensitive: i
		match any/dot matches all chars: s
		free-spacing mode: x
		unicode matching mode: u