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How do I pull n characters off the front of a string?

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Contributed by Anonymous Monk on Apr 24, 2001 at 20:03 UTC
Q&A  > strings


Description:

If I have a string that I am parsing to put into a database and the same n characters are being pulled off a string, I want to pull those chars from the string and leave the rest of the string. I want the first n chars put into $x, then removed from the string. Then the first n chars in the new string put into $y.

Answer: How do I pull n characters off the front of a string?
contributed by arturo

Bah! Why use regexes when there's a faster function designed for the purpose? =)

#assuming $n contains the number ... my $first_n = substr($string, 0, $n); #to grab $n off the end my $last_n = substr($string, -1, $n);
Answer: How do I pull n characters off the front of a string?
contributed by perlmonkey

Use substr when possible, it can be many times faster then a simple regex:

my $a = "abcdefghijklmnopqrstuvwxyz"; use Benchmark; timethese(1000000, { 's1' => sub { s1($a, 5) }, 's2' => sub { s2($a, 5) }, }); sub s1 { my $a = $_[0]; $a =~ s/^.{$_[1]}//; return $a; } sub s2 { substr($_[0], $_[1]); } Results: s1: 8 wallclock secs ( 7.95 usr + 0.09 sys = 8.04 CPU) s2: 2 wallclock secs ( 2.54 usr + 0.01 sys = 2.55 CPU)
Answer: How do I pull n characters off the front of a string?
contributed by davido

The other examples all handed off the first $n characters. But they didn't remove those first $n characters from the original $string, as the question required.

As always, there is more than one way to do it. For each of the examples below, assume the following setup:

my $string = "1234567890abcdefghijABCDEFGHIJK"; my $chars; my $n = 2;

With substr:

$chars = substr( $string, 0, $n ); substr( $string, 0, $n ) = "";

Another with substr:

( $chars, $string ) = ( substr($string,0,$n), substr($string,$n) );

And probaby the best substr solution:

$chars = substr ($string, 0, $n, "");

With a substitution s/// regexp:

$string = s/^(.{$n})(.*)$/$2/s; $chars = $1;

With split:

($chars, $string) = split /^(.{$n})/, $string, 1;

With a pattern match (m//):

($chars, $string) = $string =~ /^(.{$n})(.*)$/s;

With unpack (Not for the faint of heart):

( $chars, $string ) = unpack "a$n a@{[length($string)-$n]}", $string;

Answer: How do I pull n characters off the front of a string?
contributed by xtrem

my ($x,$y) = ($string =~ /^(.{n})(.{n})/);
This will put the 1st n char into $x and the 2nd n chars into $y without touching $string... if you want the rest of the chars:
my ($x, $y, $rest) = ($string =~ /^(.{n})(.{n})(.*)/);
if you don't like regexp's you can allways try substr

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