perlfaq4.pod

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This is sometimes referred to as putting something into "title
case", but that's not quite accurate.  Consider the proper
capitalization of the movie I<Dr. Strangelove or: How I Learned to
Stop Worrying and Love the Bomb>, for example.

Damian Conway's L<Text::Autoformat> module provides some smart
case transformations:

	use Text::Autoformat;
	my $x = "Dr. Strangelove or: How I Learned to Stop ".
	  "Worrying and Love the Bomb";

	print $x, "\n";
	for my $style (qw( sentence title highlight )) {
		print autoformat($x, { case => $style }), "\n";
		}

=head2 How can I split a [character] delimited string except when inside [character]?

Several modules can handle this sort of parsing--C<Text::Balanced>,
C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others.

Take the example case of trying to split a string that is
comma-separated into its different fields. You can't use C<split(/,/)>
because you shouldn't split if the comma is inside quotes.  For
example, take a data line like this:

	SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"

Due to the restriction of the quotes, this is a fairly complex
problem.  Thankfully, we have Jeffrey Friedl, author of
I<Mastering Regular Expressions>, to handle these for us.  He
suggests (assuming your string is contained in C<$text>):

	 @new = ();
	 push(@new, $+) while $text =~ m{
		 "([^\"\\]*(?:\\.[^\"\\]*)*)",?  # groups the phrase inside the quotes
		| ([^,]+),?
		| ,
		}gx;
	 push(@new, undef) if substr($text,-1,1) eq ',';

If you want to represent quotation marks inside a
quotation-mark-delimited field, escape them with backslashes (eg,
C<"like \"this\"">.

Alternatively, the C<Text::ParseWords> module (part of the standard
Perl distribution) lets you say:

	use Text::ParseWords;
	@new = quotewords(",", 0, $text);

=head2 How do I strip blank space from the beginning/end of a string?

(contributed by brian d foy)

A substitution can do this for you. For a single line, you want to
replace all the leading or trailing whitespace with nothing. You
can do that with a pair of substitutions.

	s/^\s+//;
	s/\s+$//;

You can also write that as a single substitution, although it turns
out the combined statement is slower than the separate ones. That
might not matter to you, though.

	s/^\s+|\s+$//g;

In this regular expression, the alternation matches either at the
beginning or the end of the string since the anchors have a lower
precedence than the alternation. With the C</g> flag, the substitution
makes all possible matches, so it gets both. Remember, the trailing
newline matches the C<\s+>, and  the C<$> anchor can match to the
physical end of the string, so the newline disappears too. Just add
the newline to the output, which has the added benefit of preserving
"blank" (consisting entirely of whitespace) lines which the C<^\s+>
would remove all by itself.

	while( <> )
		{
		s/^\s+|\s+$//g;
		print "$_\n";
		}

For a multi-line string, you can apply the regular expression
to each logical line in the string by adding the C</m> flag (for
"multi-line"). With the C</m> flag, the C<$> matches I<before> an
embedded newline, so it doesn't remove it. It still removes the
newline at the end of the string.

	$string =~ s/^\s+|\s+$//gm;

Remember that lines consisting entirely of whitespace will disappear,
since the first part of the alternation can match the entire string
and replace it with nothing. If need to keep embedded blank lines,
you have to do a little more work. Instead of matching any whitespace
(since that includes a newline), just match the other whitespace.

	$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;

=head2 How do I pad a string with blanks or pad a number with zeroes?

In the following examples, C<$pad_len> is the length to which you wish
to pad the string, C<$text> or C<$num> contains the string to be padded,
and C<$pad_char> contains the padding character. You can use a single
character string constant instead of the C<$pad_char> variable if you
know what it is in advance. And in the same way you can use an integer in
place of C<$pad_len> if you know the pad length in advance.

The simplest method uses the C<sprintf> function. It can pad on the left
or right with blanks and on the left with zeroes and it will not
truncate the result. The C<pack> function can only pad strings on the
right with blanks and it will truncate the result to a maximum length of
C<$pad_len>.

	# Left padding a string with blanks (no truncation):
	$padded = sprintf("%${pad_len}s", $text);
	$padded = sprintf("%*s", $pad_len, $text);  # same thing

	# Right padding a string with blanks (no truncation):
	$padded = sprintf("%-${pad_len}s", $text);
	$padded = sprintf("%-*s", $pad_len, $text); # same thing

	# Left padding a number with 0 (no truncation):
	$padded = sprintf("%0${pad_len}d", $num);
	$padded = sprintf("%0*d", $pad_len, $num); # same thing

	# Right padding a string with blanks using pack (will truncate):
	$padded = pack("A$pad_len",$text);

If you need to pad with a character other than blank or zero you can use
one of the following methods.  They all generate a pad string with the
C<x> operator and combine that with C<$text>. These methods do
not truncate C<$text>.

Left and right padding with any character, creating a new string:

	$padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
	$padded = $text . $pad_char x ( $pad_len - length( $text ) );

Left and right padding with any character, modifying C<$text> directly:

	substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
	$text .= $pad_char x ( $pad_len - length( $text ) );

=head2 How do I extract selected columns from a string?

(contributed by brian d foy)

If you know where the columns that contain the data, you can
use C<substr> to extract a single column.

	my $column = substr( $line, $start_column, $length );

You can use C<split> if the columns are separated by whitespace or
some other delimiter, as long as whitespace or the delimiter cannot
appear as part of the data.

	my $line    = ' fred barney   betty   ';
	my @columns = split /\s+/, $line;
		# ( '', 'fred', 'barney', 'betty' );

	my $line    = 'fred||barney||betty';
	my @columns = split /\|/, $line;
		# ( 'fred', '', 'barney', '', 'betty' );

If you want to work with comma-separated values, don't do this since
that format is a bit more complicated. Use one of the modules that
handle that fornat, such as C<Text::CSV>, C<Text::CSV_XS>, or
C<Text::CSV_PP>.

If you want to break apart an entire line of fixed columns, you can use
C<unpack> with the A (ASCII) format. by using a number after the format
specifier, you can denote the column width. See the C<pack> and C<unpack>
entries in L<perlfunc> for more details.

	my @fields = unpack( $line, "A8 A8 A8 A16 A4" );

Note that spaces in the format argument to C<unpack> do not denote literal
spaces. If you have space separated data, you may want C<split> instead.

=head2 How do I find the soundex value of a string?

(contributed by brian d foy)

You can use the Text::Soundex module. If you want to do fuzzy or close
matching, you might also try the C<String::Approx>, and
C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules.

=head2 How can I expand variables in text strings?

(contributed by brian d foy)

If you can avoid it, don't, or if you can use a templating system,
such as C<Text::Template> or C<Template> Toolkit, do that instead. You
might even be able to get the job done with C<sprintf> or C<printf>:

	my $string = sprintf 'Say hello to %s and %s', $foo, $bar;

However, for the one-off simple case where I don't want to pull out a
full templating system, I'll use a string that has two Perl scalar
variables in it. In this example, I want to expand C<$foo> and C<$bar>
to their variable's values:

	my $foo = 'Fred';
	my $bar = 'Barney';
	$string = 'Say hello to $foo and $bar';

One way I can do this involves the substitution operator and a double
C</e> flag.  The first C</e> evaluates C<$1> on the replacement side and
turns it into C<$foo>. The second /e starts with C<$foo> and replaces
it with its value. C<$foo>, then, turns into 'Fred', and that's finally
what's left in the string:

	$string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'

The C</e> will also silently ignore violations of strict, replacing
undefined variable names with the empty string. Since I'm using the
C</e> flag (twice even!), I have all of the same security problems I 
have with C<eval> in its string form. If there's something odd in
C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then
I could get myself in trouble.

To get around the security problem, I could also pull the values from
a hash instead of evaluating variable names. Using a single C</e>, I
can check the hash to ensure the value exists, and if it doesn't, I
can replace the missing value with a marker, in this case C<???> to
signal that I missed something:

	my $string = 'This has $foo and $bar';
	
	my %Replacements = (
		foo  => 'Fred',
		);

	# $string =~ s/\$(\w+)/$Replacements{$1}/g;
	$string =~ s/\$(\w+)/
		exists $Replacements{$1} ? $Replacements{$1} : '???'
		/eg;

	print $string;

=head2 What's wrong with always quoting "$vars"?

The problem is that those double-quotes force
stringification--coercing numbers and references into strings--even
when you don't want them to be strings.  Think of it this way:
double-quote expansion is used to produce new strings.  If you already
have a string, why do you need more?

If you get used to writing odd things like these:

	print "$var";   	# BAD
	$new = "$old";   	# BAD
	somefunc("$var");	# BAD

You'll be in trouble.  Those should (in 99.8% of the cases) be
the simpler and more direct:

	print $var;
	$new = $old;
	somefunc($var);

Otherwise, besides slowing you down, you're going to break code when
the thing in the scalar is actually neither a string nor a number, but
a reference:

	func(\@array);
	sub func {
		my $aref = shift;
		my $oref = "$aref";  # WRONG
		}

You can also get into subtle problems on those few operations in Perl
that actually do care about the difference between a string and a
number, such as the magical C<++> autoincrement operator or the
syscall() function.

Stringification also destroys arrays.

	@lines = `command`;
	print "@lines";     # WRONG - extra blanks
	print @lines;       # right

=head2 Why don't my E<lt>E<lt>HERE documents work?

Check for these three things:

=over 4

=item There must be no space after the E<lt>E<lt> part.

=item There (probably) should be a semicolon at the end.

=item You can't (easily) have any space in front of the tag.

=back

If you want to indent the text in the here document, you
can do this:

    # all in one
    ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
        your text
        goes here
    HERE_TARGET

But the HERE_TARGET must still be flush against the margin.
If you want that indented also, you'll have to quote
in the indentation.

    ($quote = <<'    FINIS') =~ s/^\s+//gm;
            ...we will have peace, when you and all your works have
            perished--and the works of your dark master to whom you
            would deliver us. You are a liar, Saruman, and a corrupter
            of men's hearts.  --Theoden in /usr/src/perl/taint.c
        FINIS
    $quote =~ s/\s+--/\n--/;

A nice general-purpose fixer-upper function for indented here documents
follows.  It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and
if so, strips that substring off.  Otherwise, it takes the amount of leading
whitespace found on the first line and removes that much off each
subsequent line.

    sub fix {
        local $_ = shift;
        my ($white, $leader);  # common whitespace and common leading string
        if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
            ($white, $leader) = ($2, quotemeta($1));
        } else {
            ($white, $leader) = (/^(\s+)/, '');
        }
        s/^\s*?$leader(?:$white)?//gm;
        return $_;
    }

This works with leading special strings, dynamically determined:

	$remember_the_main = fix<<'    MAIN_INTERPRETER_LOOP';
	@@@ int
	@@@ runops() {
	@@@     SAVEI32(runlevel);
	@@@     runlevel++;
	@@@     while ( op = (*op->op_ppaddr)() );
	@@@     TAINT_NOT;
	@@@     return 0;
	@@@ }
	MAIN_INTERPRETER_LOOP

Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:

	$poem = fix<<EVER_ON_AND_ON;
       Now far ahead the Road has gone,
	  And I must follow, if I can,
       Pursuing it with eager feet,
	  Until it joins some larger way
       Where many paths and errands meet.
	  And whither then? I cannot say.
		--Bilbo in /usr/src/perl/pp_ctl.c
	EVER_ON_AND_ON

=head1 Data: Arrays

=head2 What is the difference between a list and an array?

An array has a changeable length.  A list does not.  An array is
something you can push or pop, while a list is a set of values.  Some
people make the distinction that a list is a value while an array is a
variable. Subroutines are passed and return lists, you put things into
list context, you initialize arrays with lists, and you C<foreach()>
across a list.  C<@> variables are arrays, anonymous arrays are
arrays, arrays in scalar context behave like the number of elements in
them, subroutines access their arguments through the array C<@_>, and
C<push>/C<pop>/C<shift> only work on arrays.

As a side note, there's no such thing as a list in scalar context.
When you say

	$scalar = (2, 5, 7, 9);

you're using the comma operator in scalar context, so it uses the scalar
comma operator.  There never was a list there at all! This causes the
last value to be returned: 9.

=head2 What is the difference between $array[1] and @array[1]?

The former is a scalar value; the latter an array slice, making
it a list with one (scalar) value.  You should use $ when you want a
scalar value (most of the time) and @ when you want a list with one
scalar value in it (very, very rarely; nearly never, in fact).

Sometimes it doesn't make a difference, but sometimes it does.
For example, compare:

	$good[0] = `some program that outputs several lines`;

with

	@bad[0]  = `same program that outputs several lines`;

The C<use warnings> pragma and the B<-w> flag will warn you about these