Regex replacement capture followed by digit

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Use ${1} instead of $1. This is also the substitution syntax for named capturing group (?<name>).

Here’s a snippet to illustrate (see also on ideone.com):

Console.WriteLine(Regex.Replace("abc", "(.)", "$11"));        // $11$11$11
Console.WriteLine(Regex.Replace("abc", "(.)", "${1}1"));      // a1b1c1
Console.WriteLine(Regex.Replace("abc", "(?<x>.)", "${x}1"));  // a1b1c1

This behavior is explicitly documented:

Regular Expression Language Elements – Substitutions

Substituting a Numbered Group

The $number language element includes the last substring matched by the number capturing group in the replacement string, where number is the index of the capturing group.

If number does not specify a valid capturing group defined in the regular expression pattern, $number is interpreted as a literal character sequence that is used to replace each match.

Substituting a Named Group

The ${name} language element substitutes the last substring matched by the name capturing group, where name is the name of a capturing group defined by the (?<name>) language element.

If name does not specify a valid named capturing group defined in the regular expression pattern, ${name} is interpreted as a literal character sequence that is used to replace each match.

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