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1.11. Expanding and Compressing Tabs
1.11.1. Problem
You
want to convert tabs in a string to the appropriate number of spaces,
or vice versa. Converting spaces into tabs can be used to reduce file
size when the file has many consecutive spaces. Converting tabs into
spaces may be required when producing output for devices that don't
understand tabs or think them at different positions than you
do.
1.11.2. Solution
Either use a rather funny looking substitution:while ($string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e) {
# spin in empty loop until substitution finally fails
}
or use the standard Text::Tabs module:use Text::Tabs;
@expanded_lines = expand(@lines_with_tabs);
@tabulated_lines = unexpand(@lines_without_tabs);
1.11.3. Discussion
Assuming
tab stops are set every N positions (where
N is customarily eight), it's easy to convert
them into spaces. The standard textbook method does not use the
Text::Tabs module but suffers slightly from being difficult to
understand. Also, it uses the $` variable, whose
very mention currently slows down every pattern match in the program.
This is explained in Special
Variables in Chapter 6. You could use this
algorithm to make a filter to expand its input's tabstops to eight
spaces each:
while (<>) {
1 while s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
print;
}
To avoid $`, you could use a slightly more
complicated alternative that uses the numbered variables for explicit
capture; this one expands tabstops to four each instead of eight:1 while s/^(.*?)(\t+)/$1 . ' ' x (length($2) * 4 - length($1) % 4)/e;
Another approach is to use the offsets directly from the
@+ and @- arrays. This also
expands to four-space positions:1 while s/\t+/' ' x (($+[0] - $-[0]) * 4 - $-[0] % 4)/e;
If you're looking at all of these 1 while loops
and wondering why they couldn't have been written as part of a simple
s///g instead, it's because you need to
recalculate the length from the start of the line again each time
rather than merely from where the last match occurred.The convention 1 while
CONDITION is the same as while
(CONDITION) {
}, but shorter. Its origins date to when Perl ran
the first incredibly faster than the second. While the second is now
almost as fast, it remains convenient, and the habit has stuck.The standard Text::Tabs module provides
conversion functions to convert both directions, exports a
$tabstop variable to control the number of spaces
per tab, and does not incur the performance hit because it uses
$1 and $2 rather than
$& and
$`.
use Text::Tabs;
$tabstop = 4;
while (<>) { print expand($_) }
We can also use Text::Tabs to "unexpand" the tabs. This example uses
the default $tabstop value of 8:use Text::Tabs;
while (<>) { print unexpand($_) }
1.11.4. See Also
The manpage for the Text::Tabs module; the s///
operator in perlre(1) and
perlop(1); the @- and
@+ variables (@LAST_MATCH_START
and @LAST_MATCH_END) in Chapter 28 of
Programming Perl; the section on "When a global
substitution just isn't global enough" in Chapter 5 of
Programming Perl
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| 1.10. Treating a Unicode String as Octets |  | 1.12. Expanding Variables in User Input |
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