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Miko O'Sullivan


Filter::HereDocIndent - Indent here documents


 use Filter::HereDocIndent;

 # an indented block with an indented here doc
 if ($sometest) {
         print <<'(MYDOC)';

outputs (with text beginning at start of line):


HereDocIndent mimics the planned behavior of here documents in Perl 6.


Filter::HereDocIndent can be installed with the usual routine:

 perl Makefile.PL
 make test
 make install


HereDocIndent requires Filter::Util::Call, which is part of the standard distribution starting with Perl 5.6.0. For earlier versions of Perl you will need to install Filter::Util::Call, which requires either a C compiler or a pre-compiled binary.


HereDocIndent allows you to indent your here documents along with the rest of the code. The contents of the here doc and the ending delimiter itself may be indented with any amount of whitespace. Each line of content will have the leading whitespace stripped off up to the amount of whitespace that the closing delimiter is indented. Only whitespace is stripped off the beginning of the line, never any other characters

For example, in the following code the closing delimiter is indented eight spaces:

 if ($sometest) {
         print <<'(MYDOC)';

All of the content lines in the example will have the leading eight whitespace characters removed, thereby outputting the content at the beginning of the line:


If a line is indented more than the closing delimiter, it will be indented by the extra amount in the results. For example, this code (+ is used to indicate spaces):

 if ($sometest) {
 ++++++++print <<'(MYDOC)';

produces this output:


HereDocIndent does not distinguish between different types of whitespace. If you indent the closing delimiter with a single tab, and the contents eight spaces, each line of content will lose just one space character. The best practice is to be consistent in how you indent, using just tabs or just spaces.

HereDocIndent will only remove leading whitespace. If one of the lines of content is not indented, the non-whitespace characters will not be removed. The trailing newline is never removed.


By default the contents of the here document are indented to the same extent as the closing delimiter. If you want to leave the contents indented, but still indent the closing delimiter so that it lines up with its content, set the INDENT_CONTENT option to zero in when you load HereDocIndent:

 use Filter::HereDocIndent INDENT_CONTENT=>0;


BUG: Please note that there is a bug I haven't resolved with NWS filtering. If the {nws} string appears at the beginning or end of the heredoc then it's not stripped out. In the middle it should be OK.

The NWS option helps you clean up the contents of heredocs by allowing you to add whitespace in your perl code but have it stripped out when your program runs.

To enable NWS ("no whitespace") filtering, add the NWS option to the "use" command:

 use Filter::HereDocIndent NWS=>1;

Anywhere in a heredoc that HereDocIndent sees the string {nws} it will strip out that string and all surrounding whitespace. NWS is handy for outputting strings like HTML where avoiding whitespace can clutter up your code. For example, the following code will output HTML without any spaces between the tags:

        print <<"(HTML)";
        <a href="whatever.pl"> {nws}
        <img src="logo.png" alt="logo"> {nws}


HereDocIndent was written to be conservative in what it decides are here documents. HereDocIndent recognizes the most common usage for here docs and disregards other less common usages. If you constrain your here doc declarations to the format recognized by HereDocIndent (which is by far the most popular format) then your code will compile just fine.

The format recognized by HereDocIndent is a single print statement or variable assignment, followed by <<, then a quoted string or unquoted string of word characters, then a semicolon, then the end of line. Here are a few examples that would be parsed properly by HereDocIndent:

 print << '(MYDOC)';
 print << "MYDOC";
 my $var = <<EOT;
 push @arr, <<  '(MYDOC)';
 mysub (<<'MYDOC');

Here are a few examples that would not be recognized by HereDocIndent:

 push @arr, <<'MYDOC', 'foo';
 print <<'MYDOC', "------\n";

HereDocIndent does not currently recognize POD notation, so there could be unintended problems if you put text in your POD that looks like a here doc. This issue will need to be fixed in a later release. HereDocIndent also does not recognize if an entire line is inside quotes from another line, or even inside a here doc that it didn't recognize.


There are several other here doc indentation techniques, particularly those discussed in the Perl FAQ. Those techniques generally have several shortcomings.

First, they require you to modify how you create the here doc. Instead of simply creating the here doc as you usually would, except that it is indented, you have to pass the entire string into a function of through a regex to modify it.

Second, they usually require that the ending delimiter is still flush against the left margin. It should be noted that this shortcoming can be overcome by creating the heredoc delimiter with padded spaces in the left. However, even that technique requires you to ensure that the here doc declaration and the actual delimiter have matching amounts of padded space... something I personally find to be a distasteful extra drain on my brain resources. HereDocIndent allows you to simply create a delimeter and use it as usual.

Finally, many techniques either produce a string with padded spaces in the left margin, or force a function to guess how many spaces it should remove. With HereDocIndent, that information is cleanly and unambiguously determined by the indentation of the delimiter.

HereDocIndent mimics the planned behavior of here docs in Perl 6.


There have been some problems where commented out code that includes here docs causes a compiler crash. If your code won't compile check if any commented out code uses here docs. Usually to work around the problem I just put a space between the two <'s.

HereDocIndent changes the number of lines in your document, so when you get an error that includes the line number of your code, you might find that that actual problematic code is a few lines away from that line number.


Copyright (c) 2002 by Miko O'Sullivan. All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself. This software comes with NO WARRANTY of any kind.


Miko O'Sullivan miko@idocs.com


Version 0.90 August 6, 2002

Initial release

Version 0.91 November 8, 2010

Modified to fit the situation where the heredoc is an argument in a call to a function.

Minor edits to documentation.

Version 1.00 July, 2012

Added NWS option.

Minor edits to documentation.

Version 1.01 January 2, 2015

Fixed CR/LF and encoding issues with the files. Improved tests so that they have test names.