Regular Expression

A regular expression (regex or regexp for short) is a special text string for describing a search pattern. You can think of regular expressions as wildcards on steroids. You are probably familiar with wildcard notations such as *.txt to find all text files in a file manager.

? - Matches zero or one of preceding element

eg.: colou?r  =  color 

                          colour

                          coloour (wrong)

* - Matches zero or more of the preceding elements

eg.: ab*c  =  ac

                     abc

                    abbbc

                    abcdefc

+ - Matches one or more of preceding element

eg.: ab+c  =  abc

                      ac (wrong)

                      abbbc

. - Matches any single character

eg.: .at  =  hat

                  cat

                  bat

[ ] - Matches single character contained within the brackets.

eg.: [hc]  =  hat

                    cat

                    bat (wrong)

[^] - Matches a single character that is not contained within the bracket.

eg.: [^b]at  =  cat

                       hat

                       bat (wrong)

^ - Starting position within the string.

$ - ending position within string.