Check whether a string contains a substring

Question

How can I check whether a given string contains a certain substring, using Perl?

More specifically, I want to see whether s1.domain.com is present in the given string variable.

Solution 1

To find out if a string contains substring you can use the index function:

if (index($str, $substr) != -1) {
    print "$str contains $substr\n";
} 

It will return the position of the first occurrence of $substr in $str, or -1 if the substring is not found.

Solution 2

Another possibility is to use regular expressions which is what Perl is famous for:

if ($mystring =~ /s1\.domain\.com/) {
   print qq("$mystring" contains "s1.domain.com"\n);
}

The backslashes are needed because a . can match any character. You can get around this by using the \Q and \E operators.

my $substring = "s1.domain.com";
    if ($mystring =~ /\Q$substring\E/) {
   print qq("$mystring" contains "$substring"\n);
}

Or, you can do as eugene y stated and use the index function. Just a word of warning: Index returns a -1 when it can't find a match instead of an undef or 0.

Thus, this is an error:

my $substring = "s1.domain.com";
if (not index($mystring, $substr)) {
    print qq("$mystring" doesn't contains "$substring"\n";
} 

This will be wrong if s1.domain.com is at the beginning of your string. I've personally been burned on this more than once.

Solution 3

Case Insensitive Substring Example

This is an extension of Eugene's answer, which converts the strings to lower case before checking for the substring:

if (index(lc($str), lc($substr)) != -1) {
    print "$str contains $substr\n";
} 

Comments

[evgeny9]

This way is especially preferrable, when you are searching using a variable - this way you won't have to double-escape characters (in this variable string), that are special for regular expressions (like :).

[G. Cito]

I suppose index() is faster than regexps in the simple case?

[G. Cito]

An attempted perl answer to a question about removing substrings - I did not compare speed but =~, index(), ~~ and match::simple all seemed a bit awk-ward ... :-\

[BlueChips23]

You also have to make sure that the strings that you are comparing is not case sensitive. S1.DOMAIN.COM will not work for substring. But if you lc($givendomain), and then compare that with "s1.domain.com", then that will work. Also, substring is not necessarily the right approach - see my note above to eugene y's response

[avrono]

@David W. Somehow I can't get domain.com to match subdomain.domain.com using the REGEX above, any ideas ?

[melpomene]

Note that fc is recommended over lc for case-insensitive comparisons.

[Sam B]

See this as well - troubleshooters.com/codecorn/littperl/perlreg.htm