Regular expressions can be created in two different ways in JavaScript, similar to the ones used in strings. There is a more explicit definition, where you call the constructor function and pass it the pattern of your choice (and optionally any settings as well), and then, there is the literal definition, which is a shorthand for the same process. Here is an example of both (you can type this straight into the JavaScript console):

var rgx1 = new RegExp("hello");
var rgx2 = /hello/;

Both these variables are essentially the same, it's pretty much a personal preference as to which you would use. The only real difference is that with the constructor method you use a string to create an expression: therefore, you have to make sure to escape any special characters beforehand, so it gets through to the regular expression.

Besides a pattern, both forms of Regex constructors accept a second parameter, which is a string of flags. Flags are like settings or properties, which are applied on the entire expression and can therefore change the behavior of both the pattern and its methods.

var rgx1 = new RegExp("hello", "gi");
var rgx2 = /hello/gi;

Now that we have created our regular expression objects, let's use its simplest function, the test function. The test method only returns true or false, based on whether a string matches a pattern or not. Here is an example of it in action:

> var rgx = /hello/;
undefined
> rgx.test("hello");
true
> rgx.test("world");
false
> rgx.test("hello world");
true

As you can see, the first string matches and returns true, and the second string does not contain hello, so it returns false, and finally the last string matches the pattern. In the pattern, we did not specify that the string had to only contain hello, so it matches the last string and returns true.

The next method on the RegExp object, is the exec function, which, instead of just checking whether the pattern matches the text or not, exec also returns some information about the match. For this example, let's create another regular expression, and get index for the start of the pattern;

> var rgx = /world/;
undefined
> rgx.exec("world !!");
[ 'world' ]
> rgx.exec("hello world");
[ 'world' ]
> rgx.exec("hello");
null

As you can see here, the result from the function contains the actual match as the first element (rgx.exec("world !!")[0];) and if you console.dir the results, you will see it also contains two properties: index and input, which store the starting index property and complete the input text, respectively. If there are no matches, the function will return null:

Besides these two methods on the RegExp object itself, there are a few methods on the string object that accept the RegExp object as a parameter.

Using the String.replace method

The most commonly used method is the replace method. As an example, let's say we have the foo foo string and we want to change it to qux qux. Using replace with a string would only switch the first occurrence, as shown here:

In order to replace all the occurrences, we need to supply a RegExp object that has the g flag, as shown here:

> str = "hello world";
"hello world"
> str.search(/world/);
6

Using the String.match method

The last method I want to talk about right now is the match function. This function returns the same output as the exec function we saw earlier when there was no g flag (it includes the index and input properties), but returned a regular Array of all the matches when the g flag was set. Here is an example of this:

We have taken a quick pass through the most common uses of regular expressions in JavaScript (code-wise), so we are now ready to build our RegExp testing page, which will help us explore the actual syntax of Regex without combining it with JavaScript code.

The last thing we need to do is handle the form being submitted and run a regular expression. I broke the code into helper functions to help with the code flow when we go through it now. To begin with, let's write the full-click handler for the submit (Test!) button (this should go where I've inserted the comment in the script tags):

var textbox = $("#input-text");
var regexbox = $("#input-regex");
var alertbox = $("#alert-box");
var resultsbox = $("#results-box");

$("#test-button").click(function(){
  //clear page from previous run
  clearResultsAndErrors()

  //get current values
  var text = textbox.val();
  var regex = regexbox.val();

  //handle empty values
  if (text == "") {
    err("Please enter some text to test.");
  } else if (regex == "") {
    err("Please enter a regular expression.");
  } else {
    regex = createRegex(regex);

    if (!regex) {
      return;
    }

    //get matches
    var results = getMatches(regex, text);

    if (results.length > 0 && results[0] !== null) {
      var html = getMatchesCountString(results);
      html += getResultsString(results, text);
      resultsbox.html(html);
    } else {
      resultsbox.text("There were no matches.");
    }
  }
});

The first four lines select the corresponding DOM element from the page using jQuery, and store them for use throughout the application. This is a best practice when the DOM is static, instead of selecting the element each time you use it.

The rest of the code is the click handler for the submit (Test!) button. In the function that handles the Test! button, we start by clearing the results and errors from the previous run. Next, we pull in the values from the two text boxes and handle the cases where they are empty using a function called err, which we will take a look at in a moment. If the two values are fine, we attempt to create a new RegExp object and we get their results using two other functions I wrote called createRegex and getMatches, respectively. Finally, the last conditional block checks whether there were results and displays either a No Matches Found message or an element on the page that will show individual matches using getMatchesCountString to display how many matches were found and getResultsString to display the actual matches in string.

Now, let's take a look at some of these helper functions, starting with err and clearResultsAndErrors:

function clearResultsAndErrors() {
  resultsbox.text("");
  alertbox.addClass("hide").text("");
}

function err(str) {
  alertbox.removeClass("hide").text(str);
}

The first function clears the text from the results element and then hides the previous errors, and the second function un-hides the alert element and adds the error passed in as a parameter.

The next function I want to take a look at is in charge of creating the actual RegExp object from the value given in the textbox:

function createRegex(regex) {
  try {
    if (regex.charAt(0) == "/") {
      regex = regex.split("/");
      regex.shift();

      var flags = regex.pop();
      regex = regex.join("/");

      regex = new RegExp(regex, flags);
    } else {
      regex = new RegExp(regex, "g");
    }
    return regex;
  } catch (e) {
    err("The Regular Expression is invalid.");
    return false;
  }
}

If you try and create a RegExp object with flags that don't exist or invalid parameters, it will throw an exception. Therefore, we need to wrap the RegExp creation in a try/catch block, so that we can catch the error and display an error for it.

Inside the try section, we will handle two different kinds of RegExp input, the first is when you use forward slashes in your expressions. In this situation, we split this expression by forward slashes, remove the first element, which will be an empty string (the text before it is the first forward slash), and then pop off the last element which is supposed to be in the form of flags.

We then recombine the remaining parts back into a string and pass it in along with the flags into the RegExp constructor. The other case we are dealing with is where you wrote a string, and then we are simply going to pass this pattern to the constructor with only the g flag, so as to get multiple results.

The next function we have is for actually cycling through the regex object and getting results from different matches:

function getMatches(regex, text) {
  var results = [];
  var result;

  if (regex.global) {
    while((result = regex.exec(text)) !== null) {
      results.push(result);
    }
  } else {
    results.push(regex.exec(text));
  }

  return results;
}

We have already seen the exec command earlier and how it returns a results object for each match, but the exec method actually works differently, depending on whether the global flag (g) is set or not. If it is not set, it will constantly just return the first match, no matter how many times you call it, but if it is set, the function will cycle through the results until the last match returns null. In the function, the global flag is set, I use a while loop to cycle through results and push each one into the results array, whereas if it is not set, I simply call function once and push only if the first match on.

Next, we have a function that will create a string that displays how many matches we have (either one or more):

function getMatchesCountString(results) {
  if (results.length === 1) {
    return "<p>There was one match.</p>";
  } else {
    return "<p>There are " + results.length + " matches.</p>";
  }
}

Finally, we have function, which will cycle through the results array and create an HTML string to display on the page:

function getResultsString(results, text) {
  for (var i = results.length - 1; i >= 0; i--) {
    var result = results[i];
    var match  = result.toString();
    var prefix = text.substr(0, result.index);
    var suffix = text.substr(result.index + match.length);
    text = prefix 
      + '<span class="label label-info">' 
      + match 
      + '</span>' 
      + suffix;
  }
  return "<h4>" + text + "</h4>";
}

Inside function, we cycle through a list of matches and for each one, we cut the string and wrap the actual match inside a label for styling purposes. We need to cycle through the list in reverse order as we are changing the actual text by adding labels and also so as to change the indexes. In order to keep in sync with the indexes from the results array, we modify text from the end, keeping text that occurs before it, the same.

If everything goes as planned, we should now be able to test the application. For example, let's say we enter the Hello World string as the text and add the l pattern (which if you remember will be similar to entering /l/g into our application), you should get something similar to this:

Whereas, if we specify the same pattern, though without the global flag, we would only get the first match:

Of course, if you leave out a field or specify an invalid pattern, our error handling will kick in and provide an appropriate message:

With this all working as expected, we are now ready to start learning Regex by itself, without having to worry about the JavaScript code alongside it.