C# in a Nutshell, Second Edition

Cookbook Regular Expressions

To wrap up this overview of how regular expressions are used in C# applications, the following is a set of useful expressions that have been used in other environments.^[3]

• Matching roman numerals:

  string p1 = "^m*(d?c{0,3}|c[dm])"
    + "(l?x{0,3}|x[lc])(v?i{0,3}|i[vx])$";
  string t1 = "vii";
  Match m1 = Regex.Match(t1, p1);

• Swapping first two words:

  string t2 = "the quick brown fox";
  string p2 = @"(\S+)(\s+)(\S+)";
  Regex x2 = new Regex(p2);
  string r2 = x2.Replace(t2, "$3$2$1", 1);

• Matching “keyword = value” patterns:

  string t3 = "myval = 3";
  string p3 = @"(\w+)\s*=\s*(.*)\s*$";
  Match m3 = Regex.Match(t3, p3);

• Matching lines of at least 80 characters:

  string t4 = "********************"
    + "******************************"
    + "******************************";
  string p4 = ".{80,}";
  Match m4 = Regex.Match(t4, p4);

• Extracting date/time values (MM/DD/YY HH:MM:SS):

  string t5 = "01/01/01 16:10:01";
  string p5 =
    @"(\d+)/(\d+)/(\d+) (\d+):(\d+):(\d+)";
  Match m5 = Regex.Match(t5, p5);

• Changing directories (for Windows):

  string t6 =
    @"C:\Documents and Settings\user1\Desktop\";
  string r6 = Regex.Replace(t6,
    @"\\user1\\",
    @"\user2\");

• Expanding (%nn) hex escapes:

  string t7 = "%41"; // capital A
  string p7 = "%([0-9A-Fa-f][0-9A-Fa-f])";
  // uses a MatchEvaluator delegate
  string r7 = Regex.Replace(t7, p7,
    HexConvert);

• Deleting C comments (imperfectly):

  string t8 = @" /* * this is an old cstyle comment block */ "; string p8 = @" /\* # match the opening delimiter .*? # match a minimal numer ...