# Regex

**Regular expression:**

Regular expression is a pattern that matches against text.


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## **What we need to understand:**

There are a lot of such examples like URL, Phone numbers, working hours of an employee in a company and many more. So, if we are able to find a pattern out of some given text, then we can develop an expression to extract the data that matched out of whole text and that expression we call Regular Expression.


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## Anchors

^The --- matches any string that starts with "The"
end$ --- matches a string that ends with "end"
^The end$ --- exact string match (starts and ends with "The end")

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## Quantifiers — * + ? and {}

abc* --- matches a string that has "ab" followed by zero or more "c"
abc+ --- matches a string that has "ab" followed by one or more "c"
abc? --- matches a string that has "ab" followed by zero or one "c"
abc{2} --- matches a string that has "ab" followed by 2 "c"
abc{2,} --- matches a string that has "ab" followed by 2 or more "c"
abc{2,5} --- matches a string that has "ab" followed by 2 up to 5 "c"

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## OR operator — | or []

a(b|c) --- matches a string that has "a" followed by "b" or "c"
a[bc] --- same as previous

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## Character classes — \d \w \s and .

\d --- matches a single character that is a digit
\w --- matches a word character (alphanumeric character plus underscore)
\s --- matches a whitespace character (includes tabs and line breaks)
. --- matches any character
\D --- matches a single non-digit character

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## Flags

A regex usually comes within this form /abc/, where the search pattern is delimited by two slash characters /. At the end we can specify a flag with these values (we can also combine them each other):

g --- (global) does not return after the first match, restarting the subsequent searches from the end of the previous match

m --- (multi-line) when enabled ^ and $ will match the start and end of a line, instead of the whole string

i --- (insensitive) makes the whole expression case-insensitive (for instance /aBc/i would match AbC)

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## Bracket expressions — []

[a-z] --- a string that accepts only lower case letters
[0-9] --- a string that has a character from 0 to 9
[^a-zA-Z] --- a string that has not a letter from a to z or from A to Z. In this case the ^ is used as negation of the expression.

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## Grouping Constructs in Regular Expressions

Grouping constructs delineate the subexpressions of a regular expression and capture the substrings of an input string. You can use grouping constructs to do the following:

1.Match a subexpression that is repeated in the input string.
2.Apply a quantifier to a subexpression that has multiple regular expression language elements.
3.Include a subexpression in the string that is returned by the Regex.Replace and Match.Result methods.
4.Retrieve individual subexpressions from the Match.Groups property and process them separately from the matched text as a whole.

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## Email Valdiator

/^[A-Za-z]{1}[A-Za-z\d\._]+@([A-Za-z]{3,})\.([a-zA-Z]{2,})$/


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## A useful website to learn REGEX:

WWW.REGEXR.COM


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## Thanks For Listening!