Regular Expressions
A
regular expression is a set of characters that can be compared to a string to
determine whether the string meets specified format requirements. You can also
use regular expressions to extract portions of the text or to replace text.
which
uses the System.Text.RegularExpressions
namespace, performs this check using the static method System.Text.RegularExpressions.Regex.IsMatch
method
How to Use Regular
Expressions for Pattern Matching
example
:-
“^\d{5}$”
=> means that the string must
be exactly five numeric digits.
"^"
=>
Match beginning of input
"\d"
=>
Match only numeric digits
"{5}"
=> Match exactly 5
characters
"$"
=>
Match end of input
Regular
expressions can be used to match complex input patterns, too.
Regular
expressions are an extremely efficient way to check user input,
However,
Regular expressions are difficult to create unless you are extremely familiar
with the format.
"\d"
=> "[0-9]" => Matches a digit character.
"\d{3}"
=> "[0-9]{3} => Matches 3 digit character.
(<
3 and > 3 are Invalid)
"\d{2,5}"
=> "[0-9]{2,5} =>
Matches 1 to 5 digit character
(< 2 and >5 are Invalid)
"\D"
=> "[^0-9]" => Matches a nondigit character.
"\s"
=>
"[\f\n\r\t\v]" => Matches any white-space character.
(Space, Tab and
form-feed)
"\S"
=>
"[^\f\n\r\t\v]" => Matches any non-white-space character.
"\w"
=>
"[A-Za-z0-9]" => Matches any word character including underscore.
"\W"
=>
"[^A-Za-z0-9]" => Matches any nonword character, including
underscore.
How to Extract
Matched Data
Besides simply determining whether a
string matches a pattern, you can extract information from a string. For
example, if you are processing a text file that contains “Company
Name: Contoso, Inc.”, you could
extract just the company name using a regular expression.
To match a pattern and capture the
match, follow these steps:
1. Create a regular
expression, and enclose in parentheses the pattern to be matched.
2. Create an instance of
the System.Text.RegularExpressions.Match class using the static Regex.Match
method.
3. Retrieve the matched
data by accessing the elements of the Match.Groups array.
Summary
■
Regular
expressions enable you to determine whether text matches almost any type of
format. Regular expressions support dozens of special characters and operators.
The most commonly used are “^” to match the beginning of a string, “$” to match
the end of a string, “?” to make a character optional, “.” to match any
character, and “*” to match a repeated character.
■
To match
data using a regular expression, create a pattern using groups to specify the
data you need to extract, call Regex.Match to create a Match object,
and then examine each of the items in the Match.Groups array.
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