A regular expression is an object that describes a pattern of characters.
The JavaScript RegExp class represents regular expressions, and both String and RegExp define 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, as follows −
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 multi-line 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.
| Sr.No. | Expression & Description |
| 1 | […]Any one character between the brackets. |
| 2 | [^…]Any one character not between the brackets. |
| 3 | [0-9]It matches any decimal digit from 0 through 9. |
| 4 | [a-z]It matches any character from lowercase a through lowercase z. |
| 5 | [A-Z]It matches any character from uppercase A through uppercase Z. |
| 6 | [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 special character has a specific connotation. The +, *, ?, and $ flags all follow a character sequence.
| Sr.No. | Expression & Description |
| 1 | p+It matches any string containing one or more p’s. |
| 2 | p*It matches any string containing zero or more p’s. |
| 3 | p?It matches any string containing at most one p. |
| 4 | p{N}It matches any string containing a sequence of N p’s |
| 5 | p{2,3}It matches any string containing a sequence of two or three p’s. |
| 6 | p{2, }It matches any string containing a sequence of at least two p’s. |
| 7 | p$It matches any string with p at the end of it. |
| 8 | ^pIt matches any string with p at the beginning of it. |
Examples
Following examples explain more about matching characters.
| Sr.No. | Expression & Description |
| 1 | [^a-zA-Z]It matches any string not containing any of the characters ranging from a through z and A through Z. |
| 2 | p.pIt matches any string containing p, followed by any character, in turn followed by another p. |
| 3 | ^.{2}$It matches any string containing exactly two characters. |
| 4 | <b>(.*)</b>It matches any string enclosed within <b> and </b>. |
| 5 | p(hp)*It matches any string containing a p followed by zero or more instances of the sequence hp. |
Literal characters
| Sr.No. | Character & Description |
| 1 | AlphanumericItself |
| 2 | \0The NUL character (\u0000) |
| 3 | \tTab (\u0009 |
| 4 | \nNewline (\u000A) |
| 5 | \vVertical tab (\u000B) |
| 6 | \fForm feed (\u000C) |
| 7 | \rCarriage return (\u000D) |
| 8 | \xnnThe Latin character specified by the hexadecimal number nn; for example, \x0A is the same as \n |
| 9 | \uxxxxThe Unicode character specified by the hexadecimal number xxxx; for example, \u0009 is the same as \t |
| 10 | \cXThe 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 a large sum of money using the ‘\d’ metacharacter: /([\d]+)000/, Here \d will search for any string of numerical character.
The following table lists a set of metacharacters which can be used in PERL Style Regular Expressions.
| Sr.No. | Character & Description |
| 1 | .a single character |
| 2 | \sa whitespace character (space, tab, newline) |
| 3 | \Snon-whitespace character |
| 4 | \da digit (0-9) |
| 5 | \Da non-digit |
| 6 | \wa word character (a-z, A-Z, 0-9, _) |
| 7 | \Wa non-word character |
| 8 | [\b]a literal backspace (special case). |
| 9 | [aeiou]matches a single character in the given set |
| 10 | [^aeiou]matches a single character outside the given set |
| 11 | (foo|bar|baz)matches any of the alternatives specified |
Modifiers
Several modifiers are available that can simplify the way you work with regexps, like case sensitivity, searching in multiple lines, etc.
| Sr.No. | Modifier & Description |
| 1 | iPerform case-insensitive matching. |
| 2 | mSpecifies 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 |
| 3 | gPerforms a global matchthat is, find all matches rather than stopping after the first match. |
RegExp Properties
Here is a list of the properties associated with RegExp and their description.
| Sr.No. | Property & Description |
| 1 | constructorSpecifies the function that creates an object’s prototype. |
| 2 | globalSpecifies if the “g” modifier is set. |
| 3 | ignoreCaseSpecifies if the “i” modifier is set. |
| 4 | lastIndexThe index at which to start the next match. |
| 5 | multilineSpecifies if the “m” modifier is set. |
| 6 | sourceThe text of the pattern. |
In the following sections, we will have a few examples to demonstrate the usage of RegExp properties.
RegExp Methods
Here is a list of the methods associated with RegExp along with their description.
| Sr.No. | Method & Description |
| 1 | exec()Executes a search for a match in its string parameter. |
| 2 | test()Tests for a match in its string parameter. |
| 3 | toSource()Returns an object literal representing the specified object; you can use this value to create a new object. |
| 4 | toString()Returns a string representing the specified object. |
In the following sections, we will have a few examples to demonstrate the usage of RegExp methods.
