Regular Expressions in

About Regular Expressions

The .NET base class libraries provide the Regex class for processing of regular expressions.

See this article for a comparison of C# with other flavours of regular expressions.

Capture Groups

One aspect to consider is that of capture groups which return parts of the text matching various parts of the regex pattern. The approach to capture groups needs getting used to. See this documentation.

Note that there is a hierarchy where capture groups, (?<capture-name>pattern), are involved:

• where a set of zero or more matches is returned (one for each match found in the text).
• Each match contains a number of capture groups
• Each group contains a number of captures.

In addition:

• Match is derived from Group
• Group is derived from Capture

Caveats:

• The safest approach is to use named captures.
• Capture groups do generally occur in the Groups collection in the same order as they appear in the text.
• Groups[0] appears to relate to the entire match.
• Groups[<capture name>].Value is almost certainly what you are interested in. Individual captures (other than Caputures[0]) appear to be for intended for diagnostic purposes if they are present.

Regex Options

Familiarise yourself with RegexOptions.

As well as the usual IgnoreCase options there are ones for MultiLine which is useful for processing multiple lines of text in one go and IgnorePatternWhitespace which when coupled with verbatim strings, string interpolation and comments can be very expressive as shown in this illustration:

const string PREAMBLE = "preamble";
const string PWTEXT = "pwtext";
const string PW = "pw";
const string SPACE = "space";
const string POSTAMBLE = "postamble";

var pattern = $@"
    ^
    (?<{PREAMBLE}>.*)          # any text
    (?<{PWTEXT}>password)      # the literal text - password
    (?<{SPACE}>\s+)
    (?<{PW}>\w*)               # the password itself
    (?<{POSTAMBLE}>.*)         # any text
    $
  ";

var rewrittenLine = $"{grps[PREAMBLE].Value}{grps[PWTEXT].Value}{grps[SPACE].Value}{mask}{grps[POSTAMBLE].Value}";

Another option of interest is RegexOptions.Compiled for compiled regexes.