Regular Expressions – How to use?

Yesterday, my colleague Kumar Abhishek asked me to draft a Regular expression to validate url like : regularexp.indotnet.com, he was bit confused to draft the same as he wan’t aware the power of regular expression. The following lines are for Kumar Abhishek

The following are special characters when working with Regular Expressions.
They will be discussed throughout the article.

. $ ^ { [ (  ) * + ? 

 

Matching any character with dot – The Period sign [.]

The full stop or period character (.) is known as dot. It is a
wildcard that will match any character except a new line (n). For
example
if I wanted to match the ‘g’ character followed by any two characters.

Text: gau shu gnt cow
Regex: g..
Matches: gau shu gnt cow
gau
gnt

If the Singleline option is enabled, a dot matches any character
including the new line character.

 

Matching word characters – The Word sign [w]

Backslash and a lowercase ‘w’ (w) is a character class that
will match any word character. The following Regular Expression matches ‘a’
followed by two word characters.

Text: abc anaconda ant cow apple
Regex: aww
Matches: abc anaconda ant cow apple
abc
ana
ant
app

Backslash and an uppercase ‘W’ (W) will match any non-word
character.

 

Matching white-space – The Space sign [s]

White-space can be matched using s (backslash and ‘s’).
The following Regular Expression matches the letter ‘a’ followed by two word
characters then a white space character.

Text: "abc anaconda ant"
Regex: awws
Matches:
"abc "

Note that ant was not matched as it is not followed by a white space
character.

White-space is defined as the space character, new line (n),
form feed (f), carriage return (r), tab
(t) and vertical tab (v). Be careful using s as it
can lead to unexpected behaviour by matching line breaks (n and
r). Sometimes it is better to explicitly specify the characters to
match instead of using s. e.g. to match Tab and Space use
[tx0020]

 

Matching digits – The Digit sign [s]

The digits zero to nine can be matched using d (backslash and
lowercase ‘d’). For example, the following Regular Expression matches any three
digits in a row.

Text: 123 12 843 8472
Regex: ddd
Matches: 123 12 843 8472
123
843
847

 

Matching sets of single characters – The Square-Brackets sign [( )]

The square brackets are used to specify a set of single characters to match.
Any single character within the set will match. For example, the following
Regular Expression matches any three characters where the first character is
either ‘d’ or ‘a’.

Text: abc def ant cow
Regex: [da]..
Matches: abc def ant cow
abc
def
ant

The caret (^) can be added to the
start of the set of characters to specify that none of the characters in the
character set should be matched.
The following Regular Expression matches any
three character where the first character is not ‘d’ and not ‘a’.

Text: abc def ant cow
Regex: [^da]..
Matches:
"bc "
"ef "
"nt "
"cow"

 

Matching ranges of characters – The Hyphen sign [-]

Ranges of characters can be matched using the hyphen (-). the
following Regular Expression matches any three characters where the second
character is either ‘a’, ‘b’, ‘c’ or ‘d’.

Text: abc pen nda uml
Regex: .[a-d].
Matches: abc pen nda uml
abc
nda

Ranges of characters can also be combined together. the following Regular
Expression matches any of the characters from ‘a’ to ‘z’ or any digit from ‘0’
to ‘9’ followed by two word characters.

Text: abc no 0aa i8i
Regex: [a-z0-9]ww
Matches: abc no 0aa i8i
abc
0aa
i8i

The pattern could be written more simply as [a-zd]

 

Specifying the number of times to match with Quantifiers- The Plus and Star sign [+ and *]

Quantifiers let you specify the number of times that an expression must
match. The most frequently used quantifiers are the asterisk character
(*) and the plus sign (+). Note that the asterisk
(*) is usually called the star when talking about Regular
Expressions.

 

Matching zero or more times with star (*)

The star tells the Regular Expression to match the character, group, or
character class that immediately precedes it zero or more times. This
means that the character, group, or character class is optional, it can be
matched but it does not have to match. The following Regular Expression matches
the character ‘a’ followed by zero or more word characters.

Text: Anna Jones and a friend owned an anaconda
Regex: aw*
Options: IgnoreCase
Matches: Anna Jones and a friend owned an anaconda
Anna
and
a
an
anaconda

 

Matching one or more times with plus (+)

The plus sign tells the Regular Expression to match the character, group, or
character class that immediately precedes it one or more times. This
means that the character, group, or character class must be found at least once.
After it is found once it will be matched again if it follows the first match.
The following Regular Expression matches the character ‘a’ followed by at least
one word character.

Text: Anna Jones and a friend owned an anaconda
Regex: aw+
Options: IgnoreCase
Matches: Anna Jones and a friend owned an anaconda
Anna
and
an
anaconda

Note that “a” was not matched as it is not followed by any word characters.

 

Matching zero or one times with question mark (?)

To specify an optional match use the question mark (?). The
question mark matches zero or one times. The following Regular Expression
matches the character ‘a’ followed by ‘n’ then optionally followed by another
‘n’.

Text: Anna Jones and a friend owned an anaconda
Regex: an?
Options: IgnoreCase
Matches: Anna Jones and a friend owned an anaconda
An
a
an
a
an
an
a
a

 

Specifying the number of matches

The minimum number of matches required for a character, group, or character
class can be specified with the curly brackets ({n}). The
following Regular Expression matches the character ‘a’ followed by a minimum of
two ‘n’ characters. There must be two ‘n’ characters for a match to occur.

Text: Anna Jones and Anne owned an anaconda
Regex: an{2}
Options: IgnoreCase
Matches: Anna Jones and Anne owned an anaconda
Ann
Ann

A range of matches can be specified by curly brackets with two numbers inside
({n,m}). The first number (n) is the minimum
number of matches required, the second (m) is the maximum number of matches
permitted. This Regular Expression matches the character ‘a’ followed by a
minimum of two ‘n’ characters and a maximum of three ‘n’ characters.

Text: Anna and Anne lunched with an anaconda annnnnex
Regex: an{2,3}
Options: IgnoreCase
Matches: Anna and Anne lunched with an anaconda annnnnex
Ann
Ann
annn

The Regex stops matching after the maximum number of matches has been
found.

 

Matching the start and end of a string

To specify that a match must occur at the beginning of a string use the caret
character (^). For example, I want a Regular Expression pattern to
match the beginning of the string followed by the character ‘a’.

Text: an anaconda ate Anna Jones
Regex: ^a
Matches: an anaconda ate Anna Jones
"a" at position 1

The pattern above only matches the a in “an”.

Note that the caret (^) has different behaviour when used inside
the square brackets.

If the Multiline option is on, the caret (^) will match
the beginning of each line in a multiline string rather than only the start of
the string.

To specify that a match must occur at the end of a string use the dollar
character ($). If the Multiline option is on then the pattern will
match at the end of each line in a multiline string. This Regular Expression
pattern matches the word at the end of the line in a multiline string.

Text: "an anaconda
ate Anna
Jones"
Regex: w+$
Options: Multiline, IgnoreCase
Matches:
Jones

Finally, here is the Q. & Ans. fromĀ Kumar Abhishek

Q. How to write a regular expression to validate following domain name in ASP.NET Ver.1.1
wellatbell.whdev.com
Ans: <asp:regularexpressionvalidator runat=”server” controltovalidate=”txtDomainName” errormessage=”Please enter a Domain in the correct format.” validationexpression=”^([0-9a-zA-Z]).([0-9a-zA-Z]).([a-zA-Z]{3})$” cssclass=”clsForm” id=”revDomainName”>****</asp:regularexpressionvalidator>

 

Hope, the above reading will fulfill your needs. All the best.

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