Perl Regular Expression

A regular expression is a string of characters that defines a specific pattern. The perl regular expression syntax is quite similar with that of awk, grep and sed.

There are three regular expression operators inside perl:

• Matching regular expression operator
• Substitute regular expression operator
• Transliterate regular expression operator

Matching Operators

Perl matching operators have various modifiers. It is mainly used to match a string or statement to a regular expression.

The matching operator =~ is used to match a word in the given string. It is case sensitive, means if string has a lowercase letter and you are searching for an uppercase letter then it will not match.

$line = "This is rookienerd.";
if ($line =~ /java/){
   print "Matching\n";
}else{
   print "Not Matching\n";
}
if ($line =~ /Java/){
   print "Matching\n";
}else{
   print "Not Matching\n";
}

It is the opposite of the earlier one (=~). If the letters match it gives the output as not matched and vice versa.

$ line = "This is rookienerd.";
if ($line!~ /java/){
   print "Matching\n";
}else{
   print "Not Matching\n";
}
if ($line!~ /Java/){
   print "Matching\n";
}else{
   print "Not Matching\n";
}

You can also match it against a special default variable $_.

$_ = "This is rookienerd.";
    if (/java/) {
        print "Matching\n";
    }
    else {
        print "Not Matching\n";
    }
if (/Java/) {
        print "Matching\n";
    }
    else {
        print "Not Matching\n";
    }

The matching operator m is also used to match a word in the given string.

$ line = "This is rookienerd.";
if ($line=~ m[java]){
   print "Matching\n";
}else{
   print "Not Matching\n";
}
if ($line=~ m{Java}){
   print "Matching\n";
}else{
   print "Not Matching\n";
}

The $1, $2 will print the word according to the specified bracket.

my $word = "CuNaHg";
    $word =~ /(((Cu)(Na))(Hg))/;
    print "1: $1 2: $2 3: $3 4: $4 5: $5 6: $6\n";

It prints the matched character inside the bracket from a given string.

my $word = "CuNaHg";
    $word =~ /(?:(Cu)NaHg)/;
    print "$1\n"; # prints "Cu"
     $word =~ /(?:Cu(Na)Hg)/;
    print "$1\n"; # prints "Na"
     $word =~ /(?:CuNa(Hg))/;
    print "$1\n"; # prints "Hg?

Substitution Operator

The substitution operator is just an extension of the matched operator. It allows the replacement of text matched with some new text.

s/oldPattern/newPattern /;

Perl Substitution Operator with s///

Here we are replacing liquid with solid in the first part with s///.

In the second part, 'liquid' is replaced with 'solid' globally with s///g.

$line = "liquid will remain liquid until it is evaporated";
$line =~ s/liquid/solid/;
print "$line\n";
print"\n";
$line = "liquid will remain liquid until it is solidified";
$line =~ s/liquid/solid/g;
print "$line\n";

Translation Operator

Translation operator is similar as substitution operator. But translation does not use regular expression for search on replacement values.

tr/oldLetter/newLetter /;

Perl Translation Operator replacing one letter

Here, all the 'l' letters will be replaced with 'z' letters by translation operator.

$line = "liquid will remain liquid until it is evaporated";
$line =~ tr/l/z/;
print "$line\n";

Perl Translation Operator replacing more than one letter

Here, all the 'l' and 'i' letters will be replaced with 'z' and 'x' letters by translation operator.

$line = "liquid will remain liquid until it is evaporated";
$line =~ tr/li/zx/;
print "$line\n";