1. Regular Expression: Pattern Matching
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2. 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
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3. 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
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4. 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
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5. 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])
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6. 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
= ‘abcdcb’=~/.b/g; @matches will be (‘ab’,’cb’)
 Example: HTML Form, CGI
Web Programming

7. 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 ”; $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
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