Regular Expression

PCRE

• . Normally matches any character except a newline. Within square brackets the dot is literal.
• ( ) Groups a series of pattern elements to a single element. When you match a pattern within parentheses, you can use any of $1, $2, … later to refer to the previously matched pattern.
• ? Matches the preceding pattern element zero or one time.
• ? Modifies the *, +, ? or {M,N} regex that comes before to match as few times as possible.
• * Matches the preceding element zero or more times.
• {M,N} Denotes the minimum M and the maximum N match count. N can be omitted and M can be 0: {M} matches “exactly” M times; {M,} matches “at least” M times; {0,N} matches “at most” N times. x* y+ z? is thus equivalent to x{0,} y{1,}, z{0,1}.
• […] Denotes a set of possible character matches.
• | Separates alternate possibilities.
• \b Matches a zero-width boundary between a word-class character and either non-word class character or an edge; same as (^\w|\w$|\W\w|\w\W).
• \w Matches an alphanumeric character, including “_”; same as [A-Za-z0-9_] in ASCII.
• \W Matches a non-alphanumeric character, excluding “_”; same as [^A-Za-z0-9_] in ASCII.
• \s Matches a whitespace character, which in ASCII are tab, line feed, form feed, carriage return, and space.
• \S Matches anything but a whitespace.
• \d Matches a digit; same as [0-9] in ASCII.
• \D Matches a non-digit; same as [^0-9] in ASCII.
• ^ Matches the beginning of a line or string.
• $ Matches the end of a line or string.
• [^…] Matches every character except the ones inside brackets.

lookahead

(?=regex)
(?!regex)

lookbehind

(?<=regex)
(?<!regex)