Regular Expressions

Les expressions regulars són patrons utilitzats per a trobar combinacions de caràcters en cadenes de text. A JavaScript, les expressions regulars són també objectes. Aquests patrons s'utilitzen amb els mètodes exec i test de l'objecte RegExp, així com amb els mètodes match, replace, search, i split de l'objecte String. Aquest capítol descriu les expressions regulars a JavaScript.

Crear una expressió regular

Les expressions regulars es construeixen d'una de les dues maneres següents:

Utilitzant un literal d'expressió regular, el qual consisteix en un patró embolcallat en barres, tal i com es mostra en l'exemple següent:

var re = /ab+c/;

Els literals d'expressions regulars fan que l'expressió regular es compili quan es carrega l'script. Utilitzeu aquest mètode quan l'expressió regular romangui constant per a una millor eficiència.

O bé tot cridant la funció constructora de l'objecte RegExp, tal i com es mostra en l'exemple següent:

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

Utilitzar la funció constructora fa que l'expressió regular es compili en temps d'execució. Utilitzeu la funció construtora quan sapigueu d'avantmà que el patró de l'expressió regular canviarà, o bé quan no sapigueu el patró d'avantmà i l'hagueu d'obtindre a partir d'algun altre recurs, com ara preguntat-lo a l'usuari.

Escriure patrons d'expressions regulars

El patró d'una expressió regular està composat per caràcters simples, com ara /abc/, o bé una combinació de caràcters simples i especials, com ara /ab*c/ o be /Chapter (\d+)\.\d*/. L'últim exemple inclou parèntesi, els quals s'utilitzen com a dispositiu de memòria. Les coincidències trobades amb aquesta part del patró s'emmagatzemen per a un us posterior, tal i com es descriu a Utilitzar coincidències parcials amb parèntesi.

Utilitzar patrons simples

Els patrons simples estan formats per caràcters que es volen trobar dins una cadena de forma directa. Per exemple, el patró /abc/ troba coincidències en cadenes només quan es troben exactament els caràcters 'abc' junts i en aquest ordre. Es trobaria una coincidència en la cadena "Hi, do you know your abc's?" i "The latest airplane designs evolved from slabcraft". En els dos casos la coincidència és amb la subcadena 'abc'. No hi ha cap coincidència a la cadena 'Grab crab' ja que tot i que conté la subcadena 'ab c', no conté la subcadena exacta 'abc'.

Utilitzar caràcters especials

Quan la cerca de patrons requereix de quelcom més complicat que una coincidència directa, com ara trobar el caràcter b com a mínim un o més cops consecutius, o ara trobar un espai en blanc, el patró forçosament ha d'incloure caràcters especials. Per exemple, el patró /ab*c/ qualsevol combinació de caràcters en la qual un sol caràcter 'a' tingui a continuació zero o més 'b (* vol vol dir 0 o més ocurrències del caràcter previ) i una 'c' immediatament a continuació. En la cadena de caràcters "cbbabbbbcdebc," el patró anterior trobarà la coincidència 'abbbbc'.

La taula següent proporciona un llistat complet dels caràcters especials que es poden fer anar en expressions regulars, juntament amb la seva descripció.

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, "\\$&");
}

Using parentheses

Parentheses around any part of the regular expression pattern cause 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");

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);

This prints "Smith, John".

Advanced searching with flags

Regular expressions have four 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);

This displays ["fee ", "fi ", "fo "]. In this example, you could replace the line:

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

with:

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

and get the same result.

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 first) 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>