The backslash character (\) in a regular expression indicates that the character that follows it either is a special character (as shown in the following table), or should be interpreted literally. For more information, see Character Escapes.

Anchors, or atomic zero-width assertions, cause a match to succeed or fail depending on the current position in the string, but they do not cause the engine to advance through the string or consume characters. The metacharacters listed in the following table are anchors. For more information, see Anchors.

Lookarounds at a glance

When the regular expression engine hits a lookaround expression, it takes a substring reaching from the current position to the start (lookbehind) or end (lookahead) of the original string, and then runs Regex.IsMatch on that substring using the lookaround pattern. Success of this subexpression's result is then determined by whether it's a positive or negative assertion. Once they have matched, atomic groups won't be re-evaluated again, even when the remainder of the pattern fails due to the match. This can significantly improve performance when quantifiers occur within the atomic group or the remainder of the pattern.