PERL Unit 6 regular expression

Regular Expression (cont.)

Perl Programming for Linux Open Systems and the WWW

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Match Operator

Returns true or false depending on whether its regular expression matches a string

The default match target is $_ Syntax

/regular_expression/ m#regular_expression # m{regular_expression}

Match Operator

Example m/^test(.*) / /^test(.*)/ ; m#test(.*)# m{^ test(.*)} m(^ test(.*)) m>^ test(.*)>

Match Operator Example

#!/usr/bin/perl -w print ("Enter a an expression:\n"); $_=<>;chomp($_); if(m{^regular(.*)} ) { print "$_ is a valid expression\n"; }else { print "$_ is not a valid expression\n"; }

Pattern Binding Operator

Specifies the target of a match operation

Syntax $variable =~ /reg_expr/ $variable !~ /reg_expr/

Pattern Binding Operator Example

#!/usr/bin/perl -w print ("Continue ?: "); $_=<>;chomp($_); if($_ =~ /^[Yy]/) { print "Your answer is Y\n"; }else { print "Your answer is not Y \n"; }

Pattern Binding Operator

#!/usr/bin/perl -w print ("Continue ?: "); $_=<>;chomp($_); if($_ !~ /^[Yy]/) { print "Your answer is not Y\n"; }else { print "Your answer is Y \n"; }

Pattern Binding Operator

#!/usr/bin/perl -w$ans='x';while ($ans !~/^[YyNn]/){ print "Please enter 'Y'es or 'N'o: ";chomp($ans=<>);}

Pattern Binding Operator

#!/usr/bin/perl print "Enter 'Y'es : ";if (($ans = <>) =~ /^[Yy]/){ print "Yes" ;}else{ print "Not Yes"; }

Since $ans is only used once, -w was omitted so that perl will not report the error

Pattern Binding Operator

#!/usr/bin/perl $mystring = "Hello world!";print "Is there a word 'World' in the string

'$mystring'? ";if($mystring =~ m/World/i) { print "Yes"; }else{ print "No";}

Pattern Binding Operator

#!/usr/bin/perl $age = "I fear that I'll live only for 15 years.";print "$age\n";if ($age =~ /(\d*) years/) { print "I'll live only for '$1' years.\n"; }

I fear that I'll live only for 15 years.I'll live only for ‘15' years

Pattern Binding Operator

#!/usr/bin/perl $age = "I fear that I'll live only for a few

years.";print "$age\n";if ($age =~ /(\d*) years/) { print "I'll live only for '$1' years.\n"; }

Output?

Pattern Binding Operator

#!/usr/bin/perl print ("Ask me a question politely:\n");$question = <STDIN>;if ($question =~ /please/i) { print ("Thank you for being polite!\n"); } else { print ("That was not very polite!\n"); }

Substitution Operator

Replaces a string matched with a regular expression

Default target is $_ The traditional delimeters are /// --

any other single character or two pairs of bracket can be used.

Syntax s/pattern/replacement_string/“s” cannot be omitted

Substitution Operator

Example s/UNIX/Linux/ $line =~ s/Java/\.Net/ s@/usr/local/bin/perl@/usr/bin/perl@ S{/usr/bin/javac}[/usr/bin/perl]

Substitution Operator#!/usr/bin/perl $mystring = "Hello world!";print $mystring."\n";$mystring =~ s/world/letran/;print $mystring;

#!/usr/bin/perl $mystring = "Hello World!";print $mystring."\n";$mystring =~ s/world/letran/i;print $mystring;

Substitution Operator

#!/usr/bin/perl $luv="I love the whole world";print "before substitution: ";print "$luv\n";$luv =~ s/the whole world/the clear blue

skies/ ;print "after the substitution: ";print "$luv\n";

Substitution Operator

#!/usr/bin/perl $luv="I love the whole world, i love the

clear blue skies";print "before substitution: ";print "$luv\n";$luv =~ s/love/hate/g ;print "after the substitution: ";print "$luv\n";

Substitution Operator

#!/usr/bin/perl $string = "1 teng 2";$string =~ s/[a-zA-Z]+/$& x 2/e;print "$string“

The e option indicates that the replacement string, $& x 2, is to be treated as an expression

Substitution Operator

#!/usr/bin/perl $string = "its been 14 years of silence, its

been 14 years of pain";$string =~ s/(\d+)/[$1]/g;print $string;

Assignment

Create a program that counts the number of occurrences a digit appears in an input then print the sum of all the occurrences of the digit

Create a program that adds 2 to every number in an input.