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ASP.NET Regular Expressions Advanced Features

Regular expressions have two functions that have to be said, one is "naming groups" and the other is "lookaround processing". Since these features are rarely used, they are briefly described here.

Named Groups for ASP.NET Regular Expressions

Named groups allow you to name matching groups individually and then reference them in expressions using the program language. This is especially useful when combined with the Replace method to reformat the input string (by reordering, replacing elements in the input string). For example, suppose the date uses a string in MM/DD/YYYY format, and you want the date format to be DD-MM-YYYY. At this point, you can write an expression that captures *** formats, iterates through its set of matches, parses each string, and then uses string operations to build replacement strings. This requires a lot of code and a lot of processing. If you use named groups, you can accomplish the same task, as follows:

String MDYToDMY(String input) { return Regex.Replace(intput, @"\b(?\ d{1,2})/(?\ d{1,2}/(?\ d{4})\b", "${day}- ${month}-${year}"); }

You can also refer to groups by number or by name. In any case, such references are commonly referred to as "backreferences." Another common use of backreferences is in matching the expression itself, which is used to find duplicate letters: [a-z]\1. It will match "aa,""bb,""cc," but it is different from [a-z]{2} or [a-z][a-z], which are equivalent and allow matching "ab" or "ac" or any other combination of two letters. Backreferences allow an expression to remember a subset of characters in an input string that the expression has parsed and matched.

ASP.NET Regular Expression "Four-way Processing"

"Four-way processing" refers to the positive and negative Lookahead and Lookbehind features supported by many regular expression engines. Not all regular expression engines support validation four-way processing. These constructs do not use characters, even though they can match them. Some patterns may not be described without four-way processing. Especially when one part of a pattern exists that depends on another part, such a pattern cannot be described without using four-way processing. The syntax for each four-way process is described below.

syntax

description

(?=...)

Positive Lookahead

(?!...)

Negative Lookahead

(?

Negative Lookbehind

Password authentication is an example of the necessary four-way processing. Assume that in the password limit, passwords must be between 4 and 8 characters and must contain at least one number. To do this, you can test\d only in matches, and then use string operations to test length. But to do all this in regular expressions, you must use Lookahead. In particular, positive lookahead, as follows: ^(?=.*\ d). {4,8}$

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