Regular Expressions

Regular expressions are patterns used to match character combinations in strings. In JavaScript, regular expressions are also objects. These patterns are used with the exec and test methods of RegExp, and with the match, replace, search, and split methods of String. This chapter describes JavaScript regular expressions.

Creating a regular expression

You construct a regular expression in one of two ways:

Using a regular expression literal, which consists of a pattern enclosed between slashes, as follows:

var re = /ab+c/;

Regular expression literals provide compilation of the regular expression when the script is loaded. If the regular expression remains constant, using this can improve performance.

Or calling the constructor function of the RegExp object, as follows:

var re = new RegExp('ab+c');

Using the constructor function provides runtime compilation of the regular expression. Use the constructor function when you know the regular expression pattern will be changing, or you don't know the pattern and are getting it from another source, such as user input.

Writing a regular expression pattern

A regular expression pattern is composed of simple characters, such as /abc/, or a combination of simple and special characters, such as /ab*c/ or /Chapter (\d+)\.\d*/. The last example includes parentheses which are used as a memory device. The match made with this part of the pattern is remembered for later use, as described in Using parenthesized substring matches.

Using simple patterns

Simple patterns are constructed of characters for which you want to find a direct match. For example, the pattern /abc/ matches character combinations in strings only when exactly the characters 'abc' occur together and in that order. Such a match would succeed in the strings "Hi, do you know your abc's?" and "The latest airplane designs evolved from slabcraft." In both cases the match is with the substring 'abc'. There is no match in the string 'Grab crab' because while it contains the substring 'ab c', it does not contain the exact substring 'abc'.

Using special characters

When the search for a match requires something more than a direct match, such as finding one or more b's, or finding white space, the pattern includes special characters. For example, the pattern /ab*c/ matches any character combination in which a single 'a' is followed by zero or more 'b's (* means 0 or more occurrences of the preceding item) and then immediately followed by 'c'. In the string "cbbabbbbcdebc," the pattern matches the substring 'abbbbc'.

The following table provides a complete list and description of the special characters that can be used in regular expressions.

Escaping user input to be treated as a literal string within a regular expression can be accomplished by simple replacement:

function escapeRegExp(string) {
  return string.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'); // $& means the whole matched string
}

Using parentheses

Parentheses around any part of the regular expression pattern causes that part of the matched substring to be remembered. Once remembered, the substring can be recalled for other use, as described in Using Parenthesized Substring Matches.

For example, the pattern /Chapter (\d+)\.\d*/ illustrates additional escaped and special characters and indicates that part of the pattern should be remembered. It matches precisely the characters 'Chapter ' followed by one or more numeric characters (\d means any numeric character and + means 1 or more times), followed by a decimal point (which in itself is a special character; preceding the decimal point with \ means the pattern must look for the literal character '.'), followed by any numeric character 0 or more times (\d means numeric character, * means 0 or more times). In addition, parentheses are used to remember the first matched numeric characters.

This pattern is found in "Open Chapter 4.3, paragraph 6" and '4' is remembered. The pattern is not found in "Chapter 3 and 4", because that string does not have a period after the '3'.

To match a substring without causing the matched part to be remembered, within the parentheses preface the pattern with ?:. For example, (?:\d+) matches one or more numeric characters but does not remember the matched characters.

Working with regular expressions

Regular expressions are used with the RegExp methods test and exec and with the String methods match, replace, search, and split. These methods are explained in detail in the JavaScript reference.

When you want to know whether a pattern is found in a string, use the test or search method; for more information (but slower execution) use the exec or match methods. If you use exec or match and if the match succeeds, these methods return an array and update properties of the associated regular expression object and also of the predefined regular expression object, RegExp. If the match fails, the exec method returns null (which coerces to false).

In the following example, the script uses the exec method to find a match in a string.

var myRe = /d(b+)d/g;
var myArray = myRe.exec('cdbbdbsbz');

If you do not need to access the properties of the regular expression, an alternative way of creating myArray is with this script:

var myArray = /d(b+)d/g.exec('cdbbdbsbz'); // equivalent to "cdbbdbsbz".match(/d(b+)d/g);

If you want to construct the regular expression from a string, yet another alternative is this script:

var myRe = new RegExp('d(b+)d', 'g');
var myArray = myRe.exec('cdbbdbsbz');

With these scripts, the match succeeds and returns the array and updates the properties shown in the following table.

As shown in the second form of this example, you can use a regular expression created with an object initializer without assigning it to a variable. If you do, however, every occurrence is a new regular expression. For this reason, if you use this form without assigning it to a variable, you cannot subsequently access the properties of that regular expression. For example, assume you have this script:

var myRe = /d(b+)d/g;
var myArray = myRe.exec('cdbbdbsbz');
console.log('The value of lastIndex is ' + myRe.lastIndex);

// "The value of lastIndex is 5"

However, if you have this script:

var myArray = /d(b+)d/g.exec('cdbbdbsbz');
console.log('The value of lastIndex is ' + /d(b+)d/g.lastIndex);

// "The value of lastIndex is 0"

The occurrences of /d(b+)d/g in the two statements are different regular expression objects and hence have different values for their lastIndex property. If you need to access the properties of a regular expression created with an object initializer, you should first assign it to a variable.

Using parenthesized substring matches

Including parentheses in a regular expression pattern causes the corresponding submatch to be remembered. For example, /a(b)c/ matches the characters 'abc' and remembers 'b'. To recall these parenthesized substring matches, use the Array elements [1], ..., [n].

The number of possible parenthesized substrings is unlimited. The returned array holds all that were found. The following examples illustrate how to use parenthesized substring matches.

The following script uses the replace() method to switch the words in the string. For the replacement text, the script uses the $1 and $2 in the replacement to denote the first and second parenthesized substring matches.

var re = /(\w+)\s(\w+)/;
var str = 'John Smith';
var newstr = str.replace(re, '$2, $1');
console.log(newstr);

// "Smith, John"

Advanced searching with flags

Regular expressions have five optional flags that allow for global and case insensitive searching. These flags can be used separately or together in any order, and are included as part of the regular expression.

To include a flag with the regular expression, use this syntax:

var re = /pattern/flags;

or

var re = new RegExp('pattern', 'flags');

Note that the flags are an integral part of a regular expression. They cannot be added or removed later.

For example, re = /\w+\s/g creates a regular expression that looks for one or more characters followed by a space, and it looks for this combination throughout the string.

var re = /\w+\s/g;
var str = 'fee fi fo fum';
var myArray = str.match(re);
console.log(myArray);

// ["fee ", "fi ", "fo "]

You could replace the line:

var re = /\w+\s/g;

with:

var re = new RegExp('\\w+\\s', 'g');

and get the same result.

The behavior associated with the 'g' flag is different when the .exec() method is used.  (The roles of "class" and "argument" get reversed: In the case of .match(), the string class (or data type) owns the method and the regular expression is just an argument, while in the case of .exec(), it is the regular expression that owns the method, with the string being the argument.  Contrast str.match(re) versus re.exec(str).)  The 'g' flag is used with the .exec() method to get iterative progression.

var xArray; while(xArray = re.exec(str)) console.log(xArray);
// produces: 
// ["fee ", index: 0, input: "fee fi fo fum"]
// ["fi ", index: 4, input: "fee fi fo fum"]
// ["fo ", index: 7, input: "fee fi fo fum"]

The m flag is used to specify that a multiline input string should be treated as multiple lines. If the m flag is used, ^ and $ match at the start or end of any line within the input string instead of the start or end of the entire string.

Examples

The following examples show some uses of regular expressions.

Changing the order in an input string

The following example illustrates the formation of regular expressions and the use of string.split() and string.replace(). It cleans a roughly formatted input string containing names (first name last) separated by blanks, tabs and exactly one semicolon. Finally, it reverses the name order (last name first) and sorts the list.

// The name string contains multiple spaces and tabs,
// and may have multiple spaces between first and last names.
var names = 'Harry Trump ;Fred Barney; Helen Rigby ; Bill Abel ; Chris Hand ';

var output = ['---------- Original String\n', names + '\n'];

// Prepare two regular expression patterns and array storage.
// Split the string into array elements.

// pattern: possible white space then semicolon then possible white space
var pattern = /\s*;\s*/;

// Break the string into pieces separated by the pattern above and
// store the pieces in an array called nameList
var nameList = names.split(pattern);

// new pattern: one or more characters then spaces then characters.
// Use parentheses to "memorize" portions of the pattern.
// The memorized portions are referred to later.
pattern = /(\w+)\s+(\w+)/;

// New array for holding names being processed.
var bySurnameList = [];

// Display the name array and populate the new array
// with comma-separated names, last first.
//
// The replace method removes anything matching the pattern
// and replaces it with the memorized string—second memorized portion
// followed by comma space followed by first memorized portion.
//
// The variables $1 and $2 refer to the portions
// memorized while matching the pattern.

output.push('---------- After Split by Regular Expression');

var i, len;
for (i = 0, len = nameList.length; i < len; i++) {
  output.push(nameList[i]);
  bySurnameList[i] = nameList[i].replace(pattern, '$2, $1');
}

// Display the new array.
output.push('---------- Names Reversed');
for (i = 0, len = bySurnameList.length; i < len; i++) {
  output.push(bySurnameList[i]);
}

// Sort by last name, then display the sorted array.
bySurnameList.sort();
output.push('---------- Sorted');
for (i = 0, len = bySurnameList.length; i < len; i++) {
  output.push(bySurnameList[i]);
}

output.push('---------- End');

console.log(output.join('\n'));

Using special characters to verify input

In the following example, the user is expected to enter a phone number. When the user presses the "Check" button, the script checks the validity of the number. If the number is valid (matches the character sequence specified by the regular expression), the script shows a message thanking the user and confirming the number. If the number is invalid, the script informs the user that the phone number is not valid.

Within non-capturing parentheses (?: , the regular expression looks for three numeric characters \d{3} OR | a left parenthesis \( followed by three digits \d{3}, followed by a close parenthesis \), (end non-capturing parenthesis )), followed by one dash, forward slash, or decimal point and when found, remember the character ([-\/\.]), followed by three digits \d{3}, followed by the remembered match of a dash, forward slash, or decimal point \1, followed by four digits \d{4}.

The Change event activated when the user presses Enter sets the value of RegExp.input.

<!DOCTYPE html>
<html>  
  <head>  
    <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">  
    <meta http-equiv="Content-Script-Type" content="text/javascript">  
    <script type="text/javascript">  
      var re = /(?:\d{3}|\(\d{3}\))([-\/\.])\d{3}\1\d{4}/;  
      function testInfo(phoneInput) {  
        var OK = re.exec(phoneInput.value);  
        if (!OK)  
          window.alert(phoneInput.value + ' isn\'t a phone number with area code!');  
        else
          window.alert('Thanks, your phone number is ' + OK[0]);  
      }  
    </script>  
  </head>  
  <body>  
    <p>Enter your phone number (with area code) and then click "Check".
        <br>The expected format is like ###-###-####.</p>
    <form action="#">  
      <input id="phone"><button onclick="testInfo(document.getElementById('phone'));">Check</button>
    </form>  
  </body>  
</html>