When working with computer code, a task that comes up often is parsing text files.  There are many reasons to do this.  Some examples are parsing commands from an interpreted language, reading data from a dataset, reading configuration parameters, applying automated changes to code, and many more.  Text files are generally human-readable and separated into lines, so line-by-line parsing is very common.  Most programming languages have relatively easy ways to read a text file line-by-line.  The following example shows two ways to do this in perl:



$file_name = “test.txt”;


# Read the file in-place

if (open my $file, “<$file_name) {

# NOTE: The <> operator pulls the next line from a file

    while (my $line = <$file>) {

# NOTE: $line still has the newline character

        print $line;


    close $file;



# Read the entire file into a variable.  This can be faster than in-place

# but uses more memory.

if (open my $file, “<$file_name) {

# NOTE: This changes the ‘$/’ global variable so that the <> operator pulls

# in the entire file.

    my $file_contents = do { local $/; <$file> };

    my @file_lines = split (‘\R’, $file_contents);

    foreach my $line (@file_lines) {

# NOTE: This time, $line does not have a newline character

        print $line\n”;



Perl’s built-in regex features make it a very powerful tool for parsing text files.  Perl scripts can easily search for lines that match a pattern, and perform some operation when a match is found.  However, Perl is an interpreted language so it’s not suitable for projects that require large computations or real-time operation.  These tasks are better handled by a compiled language like C.  The following example shows one way to parse a text file line-by-line in C:

#include <stdio.h>


#define FILE_NAME “test.txt”


int main (int argc, char *argv[])


    FILE *fin;

// NOTE: Line length is limited

    char line[256];


    /* Open the input file for reading in text mode */

    fin = fopen(FILE_NAME, “r”);

    if (fin) {

        /* Read one line at a time until end of file */

        while (fgets(line, sizeof(line), fin)) {

// NOTE: line contains the end-of-line character

            printf(“%s”, line);



        fin = NULL;



    return 0;


The C code is a little more involved than the Perl, but not by much.  The C language does not have built-in regex support so the actual parsing could be much more complex, but such parsing may be the only way to get required data into the application.