Question

To conclude the chapter, we’ll look at a problem that calls for regular expressions. Imagine we are writing a program to automatically harvest information about our enemies from the Internet. (We will not actually write that program here, just the part that reads the configuration file. Sorry to disappoint.) The configuration file looks like this:

searchengine=http://www.google.com/search?q=$1
spitefulness=9.7

; comments are preceded by a semicolon...
; each section concerns an individual enemy
[larry]
fullname=Larry Doe
type=kindergarten bully
website=http://www.geocities.com/CapeCanaveral/11451

[gargamel]
fullname=Gargamel
type=evil sorcerer
outputdir=/home/marijn/enemies/gargamel

The exact rules for this format (which is actually a widely used format, usually called an INI file) are as follows:

• Blank lines and lines starting with semicolons are ignored.

• Lines wrapped in [ and ] start a new section.

• Lines containing an alphanumeric identifier followed by an = character add a setting to the current section.

• Anything else is invalid.

Our task is to convert a string like this into an array of objects, each with a name property and an array of settings. We’ll need one such object for each section and one for the global settings at the top.

Since the format has to be processed line by line, splitting up the file into separate lines is a good start. We used string.split("\n") to do this in Chapter 6 . Some operating systems, however, use not just a newline character to separate lines but a carriage return character followed by a newline ( "\r\n" ). Given that the split method also allows a regular expression as its argument, we can split on a regular expression like /\r?\n/ to split in a way that allows both "\n" and "\r\n" between lines.

function parseINI(string) {
  // Start with an object to hold the top-level fields
  var currentSection = {name: null, fields: []};
  var categories = [currentSection];

  string.split(/\r?\n/).forEach(function(line) {
    var match;
    if (/^\s*(;.*)?$/.test(line)) {
      return;
    } else if (match = line.match(/^\[(.*)\]$/)) {
      currentSection = {name: match[1], fields: []};
      categories.push(currentSection);
    } else if (match = line.match(/^(\w+)=(.*)$/)) {
      currentSection.fields.push({name: match[1],
                                  value: match[2]});
    } else {
      throw new Error("Line '" + line + "' is invalid.");
    }
  });

  return categories;
}

This code goes over every line in the file, updating the “current section” object as it goes along. First, it checks whether the line can be ignored, using the expression /^\s*(;.*)?$/ . Do you see how it works? The part between the parentheses will match comments, and the ? will make sure it also matches lines containing only whitespace.

If the line is not a comment, the code then checks whether the line starts a new section. If so, it creates a new current section object, to which subsequent settings will be added.

The last meaningful possibility is that the line is a normal setting, which the code adds to the current section object.

If a line matches none of these forms, the function throws an error.

Note the recurring use of ^ and $ to make sure the expression matches the whole line, not just part of it. Leaving these out results in code that mostly works but behaves strangely for some input, which can be a difficult bug to track down.

The pattern if (match = string.match(...)) is similar to the trick of using an assignment as the condition for while . You often aren’t sure that your call to match will succeed, so you can access the resulting object only inside an if statement that tests for this. To not break the pleasant chain of if forms, we assign the result of the match to a variable and immediately use that assignment as the test in the if statement.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.