Perl Example #2
Simple Matching Operations

About the Program

This program demonstrates the use of some pattern matching operations, as well as a few other new features. The responses shown in red italics show two possible forms, one if the initial input is a program in the directory, ex2.pl , and the other if it is an invalid input xxx. The perl function "chomp" allows the user to strip off extra carriage returns from standard input, which is typically the keyboard. The UNIX command "ls -l" is run inside the single backquote and the output is assigned to simple variables of scalar as well as array types.

The pattern matching process is shown using the match operator, "=~", as well as the no match approach using "!~". In the second match process, a "regular expression" is used to set up a pattern. In general, the user is searching for files that end in the appropriate ".pl" extender. The pattern looks for spaces on both sides, but does not want to include that in the actual "tagged" argument (inside the parentheses). That argument is later referenced as "$1" in after the match is found. 

#!/usr/bin/perl -w

print "Enter a file name:";
chomp($fname = <STDIN>);     # File name is read from standard input (keyboard).
                             # Chomp removes any carriage returns from input
Enter a file name: ex2.pl     or     xxx 
print "\nLooking for $fname in the following directory:\n";
Looking for ex2.pl in the following directory:

      or

Looking for xxx in the following directory:

$dir_list = `ls -l`; # run the UNIX Command - assign output to a variable print $dir_list,"\n"; 
total 31
-rw-r--r--   1 dhyatt   faculty       540 Sep  9 19:02 ex1.out
-rwxr-xr-x   1 dhyatt   faculty      1883 Sep  9 11:45 ex1.pl
-rw-r--r--   1 dhyatt   faculty         0 Sep 11 19:50 ex2.out
-rwxr-xr-x   1 dhyatt   faculty      1577 Sep 11 19:49 ex2.pl
-rw-------   1 dhyatt   faculty     24576 Sep 11 19:49 ex2.pl.swp
-rwxr-xr-x   1 dhyatt   faculty      1904 Sep  8 19:13 hello.pl

### Simple Matches ###

# Common "if" approach at end of statement using MATCH operator "=~" 

print "Found file $fname in directory.\n" if $dir_list =~ $fname;
Found file ex2.pl in directory.           ( or no response... ) 
# Familiar "if-else" construction using NO-MATCH operator "!~"

if ($dir_list !~ $fname)
     { 
      print "Sorry... No $fname in this directory.\n\n";
     } 

else {
      print "Got a Match!\n\n";
     }
Got a Match!       or       Sorry... No xxx in this directory. 
### Advanced Matching Capabilities ###

# Create an Array using the directory listing
@dir_array  = `ls -l`;

print "Here is the directory again:\n";
print @dir_array, "\n";



Here is the directory again:
total 31
-rw-r--r--   1 dhyatt   faculty       540 Sep  9 19:02 ex1.out
-rwxr-xr-x   1 dhyatt   faculty      1883 Sep  9 11:45 ex1.pl
-rw-r--r--   1 dhyatt   faculty         0 Sep 11 19:50 ex2.out
-rwxr-xr-x   1 dhyatt   faculty      1577 Sep 11 19:49 ex2.pl
-rw-------   1 dhyatt   faculty     24576 Sep 11 19:49 ex2.pl.swp
-rwxr-xr-x   1 dhyatt   faculty      1904 Sep  8 19:13 hello.pl


print "Here are the perl programs:\n";
$max_lines = $#dir_array;  #  The "$#" returns highest array index 

$pattern = '\s+(\w+\.+pl)\s';  #Define a pattern using "regular expressions"

# Meaning "\s+" - at least one or more spaces or tabs
#         "\w+" - at least one or more alpha-numeric characters 
#         "\.+" - a period or dot
#         "pl" - the proper "pl" extender
#         "\s" - a trailing space
$j=0;
for ($i=0; $i <= $max_lines; $i++) # Loop through all lines
{
    if ($dir_array[$i] =~ $pattern)
       {print $1, "\n";
        $perlprogs[$j] = $1;
        $j++;
       }
}
Here are the perl programs:
ex1.pl
ex2.pl
hello.pl
 
print "The program names are also stored in an array: ";
$, = ", ";
print @perlprogs;
print "\n";
The program names are also stored in an array: ex1.pl, ex2.pl, hello.pl

The actual program: ex2.pl

The output: ex2.out

dhyatt@thor.tjhsst.edu