here are some short examplesexpression description 'abcthe exact string abc '[abc]an abor '[ -za- ][ - ]match letter followed by digit '[ ] followed by any two characters (except newlines' +one or more ' ' *' ?any number of 'seven none zero or one ' { }exactly two ' ' { , }twothreeor four ' ' +?one or more ' taking as few as possible 'aa followed by period 'ab|zyan ab or zy '^afirst ' $last '\devery digit '\wevery letter or number '\severy whitespace '\deverything except digits '\weverything except letters and numbers '\severything except whitespace ' (?= )every followed by ' (?! )every not followed by '(?<= )aevery preceded by '(?<! )aevery not preceded by note note that in all of the examples in this we are dealing with non-overlapping patterns for instanceif we look for the pattern 'abain the string 'abababa'we see there are several overlapping matches all of our matching is done from the left and does not consider overlaps for instancewe have the followingre sub'aba ''''abababa ''*