A regular expression is an object that describes a pattern of characters.
The JavaScript RegExp class represents regular expressions, and both String and RegExpdefine methods that use regular expressions to perform powerful pattern-matching and search-and-replace functions on text.
Syntax:
A regular expression could be defined with the RegExp( ) constructor like this:
var pattern = new RegExp(pattern, attributes); or simply var pattern = /pattern/attributes; |
Here is the description of the parameters:
- pattern: A string that specifies the pattern of the regular expression or another regular expression.
- attributes: An optional string containing any of the "g", "i", and "m" attributes that specify global, case-insensitive, and multiline matches, respectively.
Brackets:
Brackets ([]) have a special meaning when used in the context of regular expressions. They are used to find a range of characters.
| Expression | Description |
|---|---|
| [...] | Any one character between the brackets. |
| [^...] | Any one character not between the brackets. |
| [0-9] | It matches any decimal digit from 0 through 9. |
| [a-z] | It matches any character from lowercase a through lowercase z. |
| [A-Z] | It matches any character from uppercase A through uppercase Z. |
| [a-Z] | It matches any character from lowercase a through uppercase Z. |
The ranges shown above are general; you could also use the range [0-3] to match any decimal digit ranging from 0 through 3, or the range [b-v] to match any lowercase character ranging from b through v.
Quantifiers:
The frequency or position of bracketed character sequences and single characters can be denoted by a special character. Each pecial character having a specific connotation. The +, *, ?, and $ flags all follow a character sequence.
| Expression | Description |
|---|---|
| p+ | It matches any string containing at least one p. |
| p* | It matches any string containing zero or more p's. |
| p? | It matches any string containing one or more p's. |
| p{N} | It matches any string containing a sequence of N p's |
| p{2,3} | It matches any string containing a sequence of two or three p's. |
| p{2, } | It matches any string containing a sequence of at least two p's. |
| p$ | It matches any string with p at the end of it. |
| ^p | It matches any string with p at the beginning of it. |
Examples:
Following examples will clear your concepts about matching chracters.
| Expression | Description |
|---|---|
| [^a-zA-Z] | It matches any string not containing any of the characters ranging from a through z and A through Z. |
| p.p | It matches any string containing p, followed by any character, in turn followed by another p. |
| ^.{2}$ | It matches any string containing exactly two characters. |
| <b>(.*)</b> | It matches any string enclosed within <b> and </b>. |
| p(hp)* | It matches any string containing a p followed by zero or more instances of the sequence hp. |
Literal characters:
| Character | Description |
|---|---|
| Alphanumeric | Itself |
| \0 | The NUL character (\u0000) |
| \t | Tab (\u0009) |
| \n | Newline (\u000A) |
| \v | Vertical tab (\u000B) |
| \f | Form feed (\u000C) |
| \r | Carriage return (\u000D) |
| \xnn | The Latin character specified by the hexadecimal number nn; for example, \x0A is the same as \n |
| \uxxxx | The Unicode character specified by the hexadecimal number xxxx; for example, \u0009 is the same as \t |
| \cX | The control character ^X; for example, \cJ is equivalent to the newline character \n |
Metacharacters
A metacharacter is simply an alphabetical character preceded by a backslash that acts to give the combination a special meaning.
For instance, you can search for large money sums using the '\d' metacharacter:/([\d]+)000/, Here \d will search for any string of numerical character.
Following is the list of metacharacters which can be used in PERL Style Regular Expressions.
Character Description . a single character \s a whitespace character (space, tab, newline) \S non-whitespace character \d a digit (0-9) \D a non-digit \w a word character (a-z, A-Z, 0-9, _) \W a non-word character [\b] a literal backspace (special case). [aeiou] matches a single character in the given set [^aeiou] matches a single character outside the given set (foo|bar|baz) matches any of the alternatives specified |
Modifiers
Several modifiers are available that can make your work with regexps much easier, like case sensitivity, searching in multiple lines etc.
| Modifier | Description |
|---|---|
| i | Perform case-insensitive matching. |
| m | Specifies that if the string has newline or carriage return characters, the ^ and $ operators will now match against a newline boundary, instead of a string boundary |
| g | Perform a global matchthat is, find all matches rather than stopping after the first match. |
RegExp Properties:
Here is a list of each property and their description.
| Property | Description |
|---|---|
| constructor | Specifies the function that creates an object's prototype. |
| global | Specifies if the "g" modifier is set. |
| ignoreCase | Specifies if the "i" modifier is set. |
| lastIndex | The index at which to start the next match. |
| multiline | Specifies if the "m" modifier is set. |
| source | The text of the pattern. |
RegExp Methods:
Here is a list of each method and its description.
| Method | Description |
|---|---|
| exec() | Executes a search for a match in its string parameter. |
| test() | Tests for a match in its string parameter. |
| toSource() | Returns an object literal representing the specified object; you can use this value to create a new object. |
| toString() | Returns a string representing the specified object. |
