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Regular Expression in JavaScript

What is a Regular Expression(RegEx)?

Regular expressions/sequence of characters are patterns used to match character combinations in strings.

A regEx can be a single character or a complicated pattern. Regular expressions can be used to perform text search and validation operations.

Special characters in regular expressions

Character Meaning
^ It Matches the beginning of an input e.g., /^A/ does not match the ‘A’ in “an A”, but matches the ‘A’ in “An E”
$ Matches the end of an input. If the multi line flag is set to true, it also matches immediately before a line break character e.g., /t$/ does not match the ‘t’ in “eater”, but matches it in “eat”
* Matches the preceding expression 0 or more times. Equivalent to {0,} e.g., /bo*/ matches ‘boooo’ in “A ghost booed” and ‘b’ in “A bird” but nothing in “A goat”
+ Matches the preceding expression 1 or more times. Equivalent to {1,} e.g., /a+/ matches the ‘a’ in “candy” and all the a’s in “caaaandy”, but nothing in “cndy”
? Matches the preceding expression 0 or 1 time e,g., /e?le?/ matches the ‘er’ in “anger”, ‘le’ in “angle” and the ‘l’ in “also”
. The decimal point matches any single character except the newline character e.g., /.n/ matches ‘an’ and ‘on’ in “nay, an apple is on the tree”, but not ‘nay’.
x(?=y) Matches ‘x’ only if it is followed by ‘y’ e.g., /Jack(?=Spat)/ matches ‘Jack’ only whn it is followed by ‘Spat’. /Jack(?=Spat|Fost)/ matches ‘Jack’ only if it is followed by ‘Spat’ or ‘Fost’; However, neither ‘Spat’ nor ‘Fost’ is part of the match results.
x(?!y) Matches ‘x’ only if it is not followed by ‘y’ e.g., /\d+(?!\.)/ matches a number only when it is not followed by a decimal point e.g., the regular expression /\d+(?!\.)/.exec(“3.141”) matches ‘141’ but not ‘3.141’
x|y Matches ‘x’ or ‘y'(when there‚Äôs no match for ‘x’)
{n} Matches exactly n occurrences of the preceding expression where N must be a positive integer. For example, /a{2}/ doesn’t match the ‘a’ in “candy,” but matches all the a’s in “caandy,” and the first two a’s in “caandy”
{n,} Matches at least n occurrences of the preceding expression e.g., /a{2,}/ matches “aa”, “aaa” and “aaaa” but not “a”
[xyz] This pattern matches any one of the characters in the brackets
[^xyz] It matches anything that is not enclosed within the brackets. Everything that works in the normal character set also works e.g., [^abc] is the same as [^a-c]. This initially match ‘r’ in “brisket” and ‘h’ in “chop”
[\b] Matches a backspace
/d Matches a digit character i.e., equivalent to [0-9] e.g., /\d/ matches ‘3’ in “B3 is the number”
/D Matches a non-digit character i.e., equivalent to [^0-9] e.g., /\D/ matches ‘B’ in “B2 is the number”
\s Matches a white space character including space, tab, form feed, line feed etc e.g., /\s\w*/ matches ‘ bar’ in “tool bar”
\S Matches a character other than white space e.g., /\S*/ matches ‘foo’ in “foo bar”
\t Matches a tab
\w Matches any alphanumeric character including underscore e.g., /\w/ matches ‘a’ in “ant,” ‘5’ in “$528,” and ‘2’ in “2D”
\W Matches any non-word character e.g., /\W/ or /[^A-Za-z0-9_]/ matches ‘%’ in “70%”
\n Where n is a positive integer, a back reference to the last substring matching the n characters in the regular expression
\xhh Matches two hexadecimal digits
\uhhh Matches four hexadecimal digits

Methods that uses regular expressions

Method Description
exec It is a RegExp method that executes a search for a match in a string. It returns an array of information or null for a mismatch
test A RegExp method that tests for a match in a string. It returns true or false
match A String method that executes a search for a match in a string. It returns an array of information or null on a mismatch
search A String method that tests for a match in a string. It returns the index of the match or -1 if the search fails
replace A String method that executes a search for a match in a string, and replaces the matched substring with a replacement substring
split A String method that uses a regular expression or a fixed string to break a string into an array of substrings.

Example

Program to check url and mail validity using regular expression

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