Perl Example #10
More on Pattern Matching
And Regular Expressions

About the Program

Code Meaning     \w     Alphanumeric Characters     \W     Non-Alphanumeric Characters     \s     White Space     \S     Non-White Space     \d     Digits     \D     Non-Digits     \b     Word Boundary     \B     Non-Word Boundary     \A   or   ^     At the Beginning of a String     \Z   or   $     At the End of a String     .     Match Any Single Character

Code Meaning     *     Zero or More Occurrences     ?     Zero or One Occurrence     +     One or More Occurrences     { N }     Exactly N Occurrences     { N,M }     Between N and M Occurrences     .* <thingy>     Greedy Match, up to the last thingy     .*? <thingy>     Non-Greedy Match, up to the first thingy     [ set_of_things ]     Match Any Item in the Set     [ ^ set_of_things ]     Does Not Match Anything in the Set     ( some_expression )     Tag an Expression   $1..$N   Tagged Expressions used in Substitutions

#!/usr/bin/perl -w

### More on Regular Expressions ###
### Pattern Matching  ###


sub print_array		# Print the full contents of the Array
{ for ($i=0; $i<=$#strings;$i++)
  {print $strings[$i], "\n";
  }
print "\n\n";
}
     
sub grep_pattern	# Print strings which contain the pattern
{ foreach (@strings)
    {print "$_\n" if /$pattern/;
     }
print "\n\n";
}

### Setting up the Array of strings

@strings = ("Two, 4, 6, Eight", "Perl is cryptic", "Perl is great"); 

@strings[3..6] = ("1, Three", "Five, 7", "Write in Perl", "Programmer's heaven");
 print_array;

## Find the word "Perl"
$pattern = 'Perl';
print "Searching for: $pattern\n";
grep_pattern;
     

## Find "Perl" at the beginning of a line
$pattern = '^Perl';
print "Searching for: $pattern\n";
grep_pattern;

     
## Find sentences that contain an "i"
$pattern = 'i';
print "Searching for: $pattern\n";
grep_pattern;

## Find words starting in "i", i.e. a space preceeds the letter
$pattern = '\si';
print "Searching for: $pattern\n";
grep_pattern;

## Find strings containing a digit
$pattern = '\d';
print "Searching for: $pattern\n";
grep_pattern;

     

## Search for a digit followed by some stuff
$pattern = '\d+.+';
print "Searching for: $pattern\n";
grep_pattern;

     
## Find strings with a digit at the end of a line
$pattern = '\d+$';
print "Searching for: $pattern\n";
grep_pattern;

## Search for a digit, possible stuff in between, and another digit
$pattern = '\d.*\d';
print "Searching for: $pattern\n";
grep_pattern;

     
## Find four-letter words, i.e. four characters offset by word boundaries
$pattern = '\b\w{4}\b';
print "Searching for: $pattern\n";
grep_pattern;

     
## Sentences with three words, three word fields separated by white space
$pattern = '\w+\s+\w+\s+\w+';
print "Searching for: $pattern\n";
grep_pattern;

     
## Find sentences with two "e" letters, and possible stuff between 
$pattern = 'e.*e';
print "Searching for: $pattern\n";
grep_pattern;

     

#### Marking Regular Expression Sub-strings and Using Substitution
     
## Substitute "Pascal" for "Perl" words at the beginning of a line
print "Substituting first Perl words.\n";
foreach(@strings)
  {s/^Perl/Pascal/g;
  }
print_array;

## Find five-letter words and replace with "Amazing"
$pattern = '\b\w{5}\b';
print "Searching for: $pattern\n";
foreach(@strings)
  {s/$pattern/Amazing/;
  }
print_array;

## Replace any "Perl" words at the end of a line with "Cobol"
print "Substituting Final Perl \n";
foreach(@strings)
  {s/Perl$/Cobol/;
  }
print_array;
     

## Delete any apostrophes followed by an "s"
print "Substituting null strings\n";
foreach(@strings)
  {s/\'s//;  # Replace with null string
  }
print_array;
     

## Search for two digits in same line, and switch their positions
print "Tagging Parts and Switching Places\n";
foreach(@strings)
  { $pattern = '(\d)(.*)(\d)';
    if (/$pattern/)
     { print "Grabbed pattern: $pattern   \$1 = $1   \$2 = $2   \$3 = $3\n";
       s/$pattern/$3$2$1/;
     }
  }

print "\n";
print_array;
     

## Marking Patterns and using multiple times
print "Expanding Patterns, and apply more than once in the same line\n";
foreach(@strings)
  { $pattern = '(\d)';
    if (/$pattern/)
     {
       s/$pattern/$1$1$1/g;
     }
  }
print "\n";
print_array;

## Marking things between word boundaries.  Using part of pattern 
print "Replacing words that end with n \n";
foreach(@strings)
  { $pattern = '\b(\w*)n\b';
    if (/$pattern/)
     { print "Grabbed pattern: $pattern   \$1 = $1   \n";
       s/$pattern/$1s/;
     }
  }
print "\n";
print_array;

## Sentences with three words, add "n't" after the middle word
$pattern = '(\w+\s+)(\w+)(\s+\w+)';
print "Searching for: $pattern\n";
foreach(@strings)
  { 
       s/$pattern/$1$2n\'t$3/;
  }
print_array;
     
     

## Sentences with either an "o" or an "e" in them 
$pattern = '[oe]';
print "Searching for: $pattern\n";
foreach(@strings)
  { 
       s/$pattern/x/g;   # The "g" modifyer means "global", or replace all 
  }                      # occurrences of the "o" or "e" found on that line.
print_array;
     
     

The actual program: exA.pl

The output: exA.out