STR31-PL. Do not pass string literals to functions expecting regexes

Many built-in functions accept a regex pattern as an argument. Furthermore, any subroutine can accept a string yet treat it as a regex pattern. Because regex patterns are encoded as regular strings, it is tempting to assume that a string literal will be treated as if a regex that matched only that string literal was supplied. This can lead to unexpected function behavior if the string contains characters that have special meanings when the string is treated as a regex pattern. Therefore, do not pass strings that are not clearly regex patterns to a function that takes a regex.

Noncompliant Code Example

This code example appears to split a list of names.

my $data = 'Tom$Dick$Harry';
my @names = split( '$', $data);

But the first argument to split() is treated as a regex pattern. Because $ indicates the end of the string, no splitting ever occurs.

Compliant Solution

This compliant solution passes a regex pattern to split() as the first argument, properly specifying $ as a raw character. Consequently, @names is assigned the three names: Tom, Dick, and Harry.

my $data = 'Tom$Dick$Harry';
my @names = split( m/\$/, $data);

Exceptions

STR31-EX0: A string literal may be passed to a function if it normally takes a regex pattern but provides special behavior for that string. For example, the perlfunc manpage [Wall 2011] says, regarding PATTERN, the first argument to split():

As a special case, specifying a PATTERN of space (' ') will split on white space just as "split" with no arguments does. Thus, "split(' ')" can be used to emulate awk's default behavior, whereas "split(/ /)" will give you as many initial null fields (empty string) as there are leading spaces.
 

Risk Assessment

Automated Detection

Bibliography

[CPAN] Elliot Shank, Perl-Critic-1.116 BuiltinFunctions::ProhibitStringySplit
[Wall 2011] perlfunc