Using Regular Expressions Metacharacters with .net – POSIX Character Classes

The POSIX or "Portable Operating System Interface for Unix" is the collective name of a family of related standards specified by the IEEE to define the application programming interface (API), along with shell and utilities interfaces for software compatible with variants of the Unix operating system, although the standard can apply to any operating system. Originally, the name stood for IEEE Std 1003.1-1988, which, as the name suggests, was released in 1988. The family of POSIX standards is formally designated as IEEE 1003 and the international standard name is ISO/IEC 9945. The standards emerged from a project that began circa 1985. Formerly known as IEEE-IX, the term POSIX was suggested by Richard Stallman in response to an IEEE request for a memorable name.

Caution..??
JavaScript does not support the use of POSIX character classes in regular expressions. All 12 POSIX classes enumerated here are generally supported in any implementation that supports POSIX. However, there may be subtle variances from the preceding descriptions.

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Using Regular Expressions Metacharacters with .net – Matching Specific Character Types

Regular Expressions Metacharacters are characters that have special meaning within regular expressions. These metacharacters are said to match classes of characters. Class metacharacters are never actually needed (you can always enumerate the characters to match or use ranges), but you will undoubtedly find them to be incredibly useful. [0-9] is a shortcut for [0123456789] and is used to match any digit. To match anything other than a digit, the set can be negated as [^0-9]. Here are some class shortcuts for digits and nondigits. Similarly for all the alphanumeric characters, A through Z (in uppercase and lowercase), the digits, the underscore, and whitespace characters.

Below table well explain us with the usability of each metacharacter for Matching Specific Character Types.

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Regular Expressions Metacharacters are characters that have special meaning within regular expressions. Character Representations metacharacters provides visually pleasing ways to match specific characters that are otherwise difficult to represent. Many utilities provide metacharacters to represent certain control characters that are sometimes machine-dependent, and which would otherwise be difficult to input or to visualize.

Below table well explain us with the usability of each metacharacter for Character Representations.

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Using Regular Expressions Metacharacters – understand escaping

Regular Expressions Metacharacters are characters that have special meaning within regular expressions. The period (.) is a metacharacter; it is used to match any single character. The star (*) is a metacharacter; it is used to match repetition of a single character any number of times. Similarly, the left bracket ([) is a metacharacter; it is used to mark the beginning of a set and the right bracket (]) is a metacharacter; it is used to mark the ending of a set.

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Regular Expressions Hexadecimal RGB color code validation with .net

Hexadecimal RGB color also known as Web colors are colors used in designing web pages. Authors of web pages have a variety of options available for specifying colors for elements of web documents. Colors may be specified as an RGB triplet in hexadecimal format (a hex triplet). A hex triplet is a six-digit, three-byte hexadecimal number used in HTML, CSS, SVG, and other computing applications, to represent colors. It uses sixteen distinct symbols, most often the symbols 0–9 to represent values zero to nine, and A, B, C, D, E, F (or a through f) to represent values ten to fifteen.

We can also use three-digit hexadecimal representation of color in the form #RGB, where RGB is a a three-digit hexadecimal which is expanded to define the six-digit color where each digit is repeated once; thus #RGB defines to the color #RRGGBB.

Regular Expression Pattern

^\#([a-fA-F0-9]{6}|[a-fA-F0-9]{3})$

A description of the regular expression:

Beginning of line or string
A numbered capture group. [[a-fA-F0-9]{6}|[a-fA-F0-9]{3}]
Select from 2 alternatives
Any character in this class: [a-fA-F0-9], exactly 6 repetitions
Any character in this class: [a-fA-F0-9], exactly 3 repetitions
End of line or string

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