# teaching machines

## CS 330 Lecture 2 – Regular expressions

### Agenda

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• Imperative languages
• Scripting languages
• Perl

### Scripting languages

John Ousterhout once said:

Scripting languages are designed for different tasks than are system programming languages, and this leads to fundamental differences in the languages. System programming languages were designed for building data structures and algorithms from scratch, starting from the most primitive computer elements such as words of memory. In contrast, scripting languages are designed for gluing: They assume the existence of a set of powerful components and are intended primarily for connecting components. System programming languages are strongly typed to help manage complexity, while scripting languages are typeless to simplify connections among components and provide rapid application development.

Scripting languages tend:

• to not be explicitly compiled,
• to not require declarations,
• to be terse,
• and to play fast and loose with types.

### Enough Perl for today

• First line is path to interpreter: #!/usr/bin/perl.
• Scalar variables are named $name. • Scalars can appear inside string literals: printf(“My name is$name.\n”). In Perl terms, such variables are interpolated.
• my($var) creates a local variable, which goes out of scope at the end of the containing block. • local($var) temporarily reassigns (to undef) a global variable. However, its value is effectively restored at the end of the containing block.
• Command-line arguments are in array @ARGV. The last index is $#ARGV. The first element is$ARGV[0].
• Reading from a file is done with the diamond operator: <STDIN> or <$in>. • If assigning a read to a scalar, exactly one input record is consumed. By default, an input record is a line. To change the input record separator, reassign$/ to undef.

### Code

#### test.pl

#!/usr/bin/perl

$var = 7; print("$var is neat.\n");

print("Number of args: ", $#ARGV + 1, "\n"); for ($i = 0; $i <=$#ARGV; ++$i) { print("$ARGV[$i]\n"); } #### base.pl #!/usr/bin/perl$line = $ARGV[0]; if ($line =~ m/^[01]+b$/) { print("You are binary!\n"); } elsif ($line =~ m/^([0-9A-Fa-f]+h|0x[0-9A-Fa-f]+)$/) { print("You are hexadecimal!\n"); } else { print("You are something else!\n"); } #### states.txt Alabama - Montgomery Alaska - Juneau Arizona - Phoenix Arkansas - Little Rock California - Sacramento Colorado - Denver Connecticut - Hartford Delaware - Dover Florida - Tallahassee Georgia - Atlanta Hawaii - Honolulu Idaho - Boise Illinois - Springfield Indiana - Indianapolis Iowa - Des Moines Kansas - Topeka Kentucky - Frankfort Louisiana - Baton Rouge Maine - Augusta Maryland - Annapolis Massachusetts - Boston Michigan - Lansing Minnesota - St. Paul Mississippi - Jackson Missouri - Jefferson City Montana - Helena Nebraska - Lincoln Nevada - Carson City New Hampshire - Concord New Jersey - Trenton New Mexico - Santa Fe New York - Albany North Carolina - Raleigh North Dakota - Bismarck Ohio - Columbus Oklahoma - Oklahoma City Oregon - Salem Pennsylvania - Harrisburg Rhode Island - Providence South Carolina - Columbia South Dakota - Pierre Tennessee - Nashville Texas - Austin Utah - Salt Lake City Vermont - Montpelier Virginia - Richmond Washington - Olympia West Virginia - Charleston Wisconsin - Madison Wyoming - Cheyenne #### stater.pl #!/usr/bin/perl open($in, '<', $ARGV[0]) or die("Couldn't open file. Boo hoo!"); print("State states[] = {\n"); while ($line = <$in>) {$line =~ m/(.*) - (.*)/;
print("  new State(\"$1\", \"$2\"),\n");
}
print("};\n");

close(\$in);

• Homework 1

### Haiku

Doctor, mechanic–
Jobs of my dream family!
Dentist, regexpert…