The static Replace methods are equivalent to constructing a Regex object with the specified regular expression pattern and calling the instance method Replace.
The pattern parameter consists of various regular expression language elements that symbolically describe the string to match. For more information about regular expressions, see .NET Framework Regular Expressions and Regular Expression Language Elements. The search for matches starts at the beginning of the input parameter string.
Substitutions are allowed only within a replacement pattern. For similar functionality within a regular expression, use a backreference such as \1.
Character escapes and substitutions are the only special constructs recognized in a replacement pattern. All other syntactic constructs are allowed in regular expressions only and not recognized in replacement patterns. For example, the replacement pattern a*${test}b inserts the string "a*" followed by the substring matched by the "test" capturing group, if any, followed by the string "b". The * character is not recognized as a metacharacter within a replacement pattern. Similarly, $-patterns are not recognized within a regular expression matching pattern. Within a regular expression, $ denotes the end of the string. Other examples are: $123 substitutes the last substring matched by group number 123 (decimal), and ${name} substitutes the last substring matched by a (?<name>) group.