POE::Filter::Line(3) User Contributed Perl Documentation POE::Filter::Line(3)

POE::Filter::Line - serialize and parse terminated records (lines)

use POE qw(Wheel::FollowTail Filter::Line);
  inline_states => {
    _start => sub {
      $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
        Filename => "/var/log/system.log",
        InputEvent => "got_log_line",
        Filter => POE::Filter::Line->new(),
    got_log_line => sub {
      print "Log: $_[ARG0]\n";

POE::Filter::Line parses stream data into terminated records. The default parser interprets newlines as the record terminator, and the default serializer appends network newlines (CR/LF, or "\x0D\x0A") to outbound records.

Record terminators are removed from the data POE::Filter::Line returns.

POE::Filter::Line supports a number of other ways to parse lines. Constructor parameters may specify literal newlines, regular expressions, or that the filter should detect newlines on its own.


POE::Filter::Line's new() method has some interesting parameters.

new() accepts a list of named parameters.

In all cases, the data interpreted as the record terminator is stripped from the data POE::Filter::Line returns.

"InputLiteral" may be used to parse records that are terminated by some literal string. For example, POE::Filter::Line may be used to parse and emit C-style lines, which are terminated with an ASCII NUL:

my $c_line_filter = POE::Filter::Line->new(
  InputLiteral => chr(0),
  OutputLiteral => chr(0),

"OutputLiteral" allows a filter to put() records with a different record terminator than it parses. This can be useful in applications that must translate record terminators.

"Literal" is a shorthand for the common case where the input and output literals are identical. The previous example may be written as:

my $c_line_filter = POE::Filter::Line->new(
  Literal => chr(0),

An application can also allow POE::Filter::Line to figure out which newline to use. This is done by specifying "InputLiteral" to be undef:

my $whichever_line_filter = POE::Filter::Line->new(
  InputLiteral => undef,
  OutputLiteral => "\n",

"InputRegexp" may be used in place of "InputLiteral" to recognize line terminators based on a regular expression. In this example, input is terminated by two or more consecutive newlines. On output, the paragraph separator is "---" on a line by itself.

my $paragraph_filter = POE::Filter::Line->new(
  InputRegexp => "([\x0D\x0A]{2,})",
  OutputLiteral => "\n---\n",

"MaxBuffer" sets the maximum amount of data that the filter will hold onto while trying to find a line ending. Defaults to 512 MB.

"MaxLength" sets the maximum length of a line. Defaults to 64 MB.

If either the "MaxLength" or "MaxBuffer" constraint is exceeded, "POE::Filter::Line" will throw an exception.


POE::Filter::Line has no additional public methods.

POE::Filter::Line exports the FIRST_UNUSED constant. This points to the first unused element in the $self array reference. Subclasses should store their own data beginning here, and they should export their own FIRST_UNUSED constants to help future subclassers.

Please see POE::Filter for documentation regarding the base interface.

The SEE ALSO section in POE contains a table of contents covering the entire POE distribution.

The default input newline parser is a regexp that has an unfortunate race condition. First the regular expression:


While it quickly recognizes most forms of newline, it can sometimes detect an extra blank line. This happens when a two-byte newline character is broken between two reads. Consider this situation:

some stream dataCR
LFother stream data

The regular expression will see the first CR without its corresponding LF. The filter will properly return "some stream data" as a line. When the next packet arrives, the leading "LF" will be treated as the terminator for a 0-byte line. The filter will faithfully return this empty line.

It is advised to specify literal newlines or use the autodetect feature in applications where blank lines are significant.

Please see POE for more information about authors and contributors.

2024-07-13 perl v5.38.2