Using Regular Expressions Metacharacters with .net – Lookahead and Lookbehind Zero-Width Assertions

Perl 5 introduced two very powerful constructs: "lookahead" and "lookbehind". Collectively, these are called "lookaround". They are zero-width just like other "zero-width assertions" don’t match actual text, but rather positions in the text. The difference is that lookarounds will actually match characters, but then give up the match and only return the result: match or no match. That is why they are called "assertions". They do not consume characters in the string, but only assert whether a match is possible or not. Lookarounds allow you to create regular expressions that are impossible to create without them, or that would get very longwinded without them.

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Using Regular Expressions Metacharacters with .net – Anchors and Other "Zero-Width Assertions"

Anchors and other "zero-width assertions" don’t match actual text, but rather positions in the text.

^, \A

Caret (^) matches at the beginning of the text being searched, and, if in an enhanced line-anchor match mode, after any newline. In some systems, an enhanced-mode ^ can match after Unicode line terminators, as well. While \A always matches only at the start of the text being searched, regardless of any match mode.

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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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