Perl Best Practices [Electronic resources] نسخه متنی

12.24. String Comparisons

Prefer fixed-string eq comparisons to fixed-pattern regex matches .

If you're trying to compare a string against a fixed number of fixed keywords, the temptation is to put them all inside a single regex, as anchored alternatives:

# Quit command has several variants...
last COMMAND if $cmd =~ m{\A (?: q | quit | bye ) \z}xms;

The usual rationale for this is that a single, highly optimized regex match must

surely be quicker than three separate eq tests:

# Quit command has several variants...

last COMMAND if $cmd eq 'q'
|| $cmd eq 'quit'
|| $cmd eq 'bye'; 

Unfortunately, that's not the case. Regex-matching against a series of fixed alternations is at least 20% slower than individually eq-matching the same stringsnot to mention the fact that the eq-based version is significantly more readable.

Likewise, if you're doing a pattern match merely to get case insensitivity:

# Quit command is case-insensitive...
last COMMAND if $cmd =~ m{\A quit \z}ixms;

then it's more efficient, and arguably more readable, to write:

# Quit command is case-insensitive...

last COMMAND if lc($cmd) eq 'quit'; 

Sometimes, if there are a large number of possibilities to test:

Readonly my @EXIT_WORDS => qw(
q  quit  bye  exit  stop  done  last  finish  aurevoir
); 

or the number of possibilities is indeterminate at compile time:

Readonly my @EXIT_WORDS
=> slurp $EXIT_WORDS_FILE, {chomp=>1}; 

then a regex might seem like a better alternative, because it can easily be built on the fly:

Readonly my $EXIT_WORDS => join '|', @EXIT_WORDS;
# Quit command has several variants...
last COMMAND if $cmd =~ m{\A (?: $EXIT_WORDS ) \z}xms;

But, even in these cases, eq offers a cleaner (though now slower) solution:

use List::MoreUtils  qw( any );
# Quit command has several variants...

last COMMAND if any { $cmd eq $_ } @EXIT_WORDS; 

Of course, in this particular case, an even better solution would be to use table look-up instead:

Readonly my %IS_EXIT_WORD
=> map { ($_ => 1) } qw(
q  quit  bye  exit  stop  done  last  finish  aurevoir
);
# and later...
# Quit command has several variants...

last COMMAND if $IS_EXIT_WORD{$cmd};