Regular Expression: Pattern Matching Web Programming

Introduction What is Regular Expression? Pattern matching Grammar /rule for matching pattern in strings pattern = a sequence of characters Pattern matching Syntax $target =~ /$pattern/; searches $target for $pattern true if found, false otherwise ( “prog1.pl” =~ /pl/) : true ( “prog1.pl” =~ /PL/) : false can use any pattern delimiter character with m e.g. $target =~ m!$pattern! (or m|$pattern|) $target !~ /$pattern/; true if NOT found, false otherwise  Example script Web Programming

Matching Repeats .  Match any character +  Match one or more preceding characters e.g. /ab+c/ matches ‘abc’, ‘abbc’, ‘abbbc’, etc. *  Match zero or more preceding characters e.g. /ab*c/ matches ‘ac’, ‘abc’, ‘abbc’, ‘abbbc’, etc. ?  Match zero or one preceding character e.g. /ab?c/ matches ‘ac’, or ‘abc’. .  Match any character e.g. /b.a/ matches ‘abba’, ‘b1a’, etc. /b.*a/ matches ‘aba’, ‘abba’, ‘banana’, etc. ‘.’ does not match newline (i.e. \n) /b.*a/ does not match ‘ab\na’, ‘abb\na’, etc.  Regular Expression Checker: Perl script, HTML Form, CGI Web Programming

Special Characters [$pattern]  Match any character in $pattern e.g. /[Rr]ed/ matches ‘bredd’, ‘bRedd’, ‘red’, ‘Red’, etc. /[0-9]/ matches any number /[a-zA-Z]/ matches any alphabet /[0-9a-zA-z]/ matches any alphanumeric [^$pattern]  Match any character except those in $pattern e.g. /[^0-9]/ matches any non-numeric characters. \b  Match at word boundary Any character other than alphanumeric and underscore (i.e., [0-9a-zA-Z_]) e.g. /\bred\b/ matches ‘is red’, ‘red rose’, ‘$red’  Example: HTML Form, CGI Web Programming

Escape Sequences \d \D \w \W \s \S any digit (i.e. [0-9] ) any non-digit (i.e. [^0-9]) \w any word character (i.e. [_0-9a-zA-Z]) \W any non-word character (i.e. [^_0-9a-zA-Z]) \s any white space (i.e. [ \r\t\n\f]) \S any non-white space (i.e. [^ \r\t\n\f]) Web Programming

Pattern Matching Options /$pattern/i Ignore case e.g. /ab/i matches ‘ab’, ‘AB’, ‘Ab’, ‘aB’ /$pattern/g Matches all possible patterns Returns a list of matches e.g. @matches = ‘abcdcb’=~/.b/g; @matches will be (‘ab’,’cb’)  Example: HTML Form, CGI Web Programming

Substitution $string =~ s/$pattern/$replacement/; Substitution Options Replace $pattern with $replacement in $string $string = “Before substitution”; $string =~ s/Before/After/;  “After substitution” Substitution Options $string =~ s/$pattern/$replacement/i; Ignore case of $pattern $string = “One plus one is done.”; $string =~ s/one/ONE/i;  “ONE plus one is done.” $string =~ s/$pattern/$replacement/g; Change all occurrence of $pattern in $string $string = “ this is line 1. ”; $string =~ s/ +/ /g;  “ this is a line. ” $string =~ s/$pattern/$replacement/s; Treat $string as a single line (i.e., . will match \n) $string = “this <img src=\n img.gif>image”; $string =~ s/<.+>//s;  “this image”  Example Web Programming