Recently, I tried diffing two large files only for the UNIX diff command to choke the OS. That's because the diff utility slurps both files, requiring 2x memory consumption.

I thought to provide chunking variants and measure the time taken for a 99 MB file.

Perl

#!/usr/bin/env perl
#use v5.20;
#use feature qw(signatures);
#no warnings qw(experimental::signatures);

use v5.36;
use autodie;

exit 1 if not @ARGV;

sub read_file ($fh, $chunk_size=65536) {
    # Return the next chunk, including to the end of line.
    read($fh, my $chunk, $chunk_size);
    if (length($chunk) && substr($chunk, -1) ne "\n") {
        return $chunk if eof($fh);
        $chunk .= readline($fh);
    }
    return $chunk;
}

my $mul_pattern = 'mul\(\d{1,3},\d{1,3}\)';
my $filename = shift;
my $count = 0;

if (open(my $fh, '<', $filename)) {
    while (length(my $chunk = read_file($fh))) {
        $count += () = $chunk =~ m/$mul_pattern/g;
    }
}

print "Found $count matches.\n";

[download]

Python

#!/usr/bin/env python
import re, sys

if len(sys.argv) < 2: sys.exit(1)

def read_file (file, chunk_size=65536):
    """
    Lazy function generator to read a file in chunks,
    including to the end of line.
    """
    while True:
        chunk = file.read(chunk_size)
        if not chunk:
            break
        if not chunk.endswith('\n'):
            chunk += file.readline()
        yield chunk

mul_re = re.compile(r"mul\(\d{1,3},\d{1,3}\)")
filename = sys.argv[1]
count = 0

try:
    with open (filename, "r") as file:
        for chunk in read_file(file):
            count += len(mul_re.findall(chunk))
except Exception as e:
    print(e, file=sys.stderr)
    sys.exit(1)

print(f"Found {count} matches.")

[download]

Results

Perl   0.463s
Found 200246 matches.

Python 0.250s
Found 200246 matches.

[download]

In reply to Re: Python regex faster than Perl? - Chunking by marioroy
in thread Python regex faster than Perl? by dave93

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