Character and Position Matching a matches the single character "a" . matches any character \. matches only "." and no other character ^ matches the beginning of the text $ matches the end of the text [[:<:]] matches the point between the last non-word character and the beginning of a word [[:>:]] matches the point between the last character of a word and the following non-word character Examples gigo matches "gigo", "gigoit" or "gigoit.org" ^gigo matches "gigoit" but not "http://www.gigoit.org" it$ matches "gigoit" but not "http://www.gigoit.org" . matches any single character .n matches "an", "on", "3n", etc. [[:<:]]it matches "it" but not "gigoit" (no letters before "it") gigo[[:>:]] matches "gigo" but not "gigoit" (no letters after "gigo") [[:<:]]gigo[[:>:]] matches "gigo" but not "gigoit" or "www.gigoit.org" Character Class [ begins a character class ] ends a character class ^ used inside square brackets, it negates the character class - used inside square brackets, it separates the beginning and end of a range Examples [abc] matches one of "a", "b", or "c" [abc][abc] matches the "ac" in "action", but not any part of "auction" [Gg]igoit matches "Gigoit" or "gigoit" gigo[^it] matches "gigo" or "gigo it " or "gigo-it", but not "gigoit" [x-zX-Z] matches one of "x", "y", "z", "X", "Y" or "Z" Alternation | matches nothing on its own, but lets the regex match if either the pattern before or after matches Examples gigo|it matches any string containing either "gigo" or "it" GIGOIT|gigoit matches "GIGOIT" or "gigoit", but not "Gigoit" GIGOIT|gigoit|Gigoit matches any of "GIGOIT", "gigoit", or "Gigoit" Repetition Operators ? matches zero or one of the preceding character or sequence * matches zero or more (any number) of the preceding character or sequence + matches one or more of the preceding character or sequence Examples gigo-?it matches "gigoit" or "gigo-it" [Gg]igo-?it matches "Gigoit", "Gigo-it", "gigoit", or "gigo-it" app.*s matches "apps", "apples", "applications" , "application forms", etc. cho+se matches "chose" or "choose" (or "choooooose") Sequence Grouping ( matches nothing on its own; delimits the beginning of a sequence ) matches nothing on its own; delimits the end of a sequence Examples gigo(it)? matches "gigo" or "gigoit" but not "gigo-it" (gigo|Gigo)[^it] matches "gigo" or "Gigo", but not "gigoit" or "Gigoit" (Gigo.*|gigo.*).(Org|org) "matches "gigoit.org" or "gigoit.org", among others