JavaScript Regular Expressions


Regular expressions in JavaScript

The combinations of characters in a regular expression forms a search pattern that is used to match combinations of characters in strings. In JavaScript, regular expressions are also objects. The patterns are used with the exec and test methods of RegExp, and with the matchreplacesearch, and split methods of String

You use the seach pattern to describe what your are searching for in a string and the expressions are used to do all kind of text search and text replace operations.


How to use a simple pattern

Let us check out a very simple example. Here we are searching for the patten 'Bu'.

 
text = "Bumblebee"; 
alert(/Bu/.exec(text)); //Expected result: 'Bu'

Characters that can be used in regular expressions

The following table does not show the full list of characters that can be used in regular expressions but rather an excerpt of all existing characters that can be used.



\

When a backslash is placed before a non-special character it tells you that the next character is special and shouldn't interpreted literally. 

^ Matches beginning of input. If the multiline flag is set to true, also matches immediately after a line break character.

It will try to match the beginning of the input. For example, /^B/ does not match the 'B' in "an Bumblebee", but does match the 'B' in "Bumblebee fur".
$

Matches end of input. If the multiline flag is set to true, also matches immediately before a line break character.

It will try to match the end of the input. For example, /n$/ will not match the 'n' in "runner", but it matches it in "run".

*

It will match the preceding expression 0 or more times.

 

+

Matches the preceding expression 1 or more times.

For example, /n+/ matches the 'n' in "hunk" and all n in "honneybag".

? Will find the expression 0 or 1 time. 
For example, /e?le?/ matches the 'el' in "angel" and the 'le' in "angle" and also the 'e' in "Berlin".
.

The decimal point will find any single character except for the newline character.

\d

Search for a digit

\uxxxx Search for the Unicode character that is specified by the hexadecimal number xxxx
x(?=y)

This one is call a lookahed and find 'x' only if 'x' is followed by 'y'. This is called a lookahead.

(x|y)

Find any of the alternatives separated with |.
For example, /green|red/ matches 'green' in "green apple" and 'red' in "red apple."

\s

Search for a whitespace character.

\b Will try to search for a match on the first- or the last character of a string
x? It will match any strings that contain zero or one occurrences of x
x+ It will match any strings that contain one ore more x
x* It will match any strings that contain zero or more occurrences of x

Learn more about JavaScript Operators Learn more about JavaScript Comparisons

Parentheses Delimiter

If some parts of the pattern in the regular expression is delimited by parentheses (...), then that part of the pattern becomes a separate element of the array.


JS JavaScript - Regular Expressions - Parentheses Delimiter

var testString = "JavaScript can be used to many things...";
 
var result = testString.match( /JAVA(SCRIPT)/i );
 
alert( result[0] ); // Result: JavaScript - The complete match.
alert( result[1] ); // Result: Script - Parts of the match.
alert( result.input ); // Result: The input string.

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JavaScript is one of the 3 core language technologies of the World Wide Web that all web developers must learn:

1.      HTML to define the content of web pages.

2.      CSS to specify the layout of web pages.

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