Interpreting Regular Expressions in C #

In this article Article . Text. classes and methods in regularexpression, examples of string matching and replacement, and detailed information about the group structure. Finally, we will introduce some common expressions that you may use. Basic knowledge to be mastered The knowledge of Rule expressions may be one of the things that many programmers often forget. In this article, we will assume that you have mastered the usage of regular expressions, especially the expressions in Perl 5 .. Net Regexp class is a superset of expressions in Perl 5, So theoretically it will be a good start point. We also assume that you have basic knowledge of C # syntax and. NET architecture. If you do not have knowledge about Rule expressions, I suggest you start with the Perl 5 syntax. The authoritative book on Rule expressions is written by Jeffrey fredel. for readers who want to have a deep understanding of expressions, we strongly recommend that you read Read this book. Regularexpression combination The Regexp rule class is included in System. text. regularexpressions. in the DLL file, you must reference this file when compiling the application software. For example, csc r: system. text. regularexpressions. the DLL foo.cscommand creates the foo.exe file, which references system. text. regularexpressions file. Namespace Introduction The namespace contains only six classes and one definition. They are: Capture: contains a matching result; Capturecollection: the sequence of capture; Group: the result of a group record, inherited by capture; Match: the matching result of an expression, inherited by the Group; Matchcollection: a sequence of match; Matchevaluator: the proxy used to perform the replacement operation; RegEx: An Example of the compiled expression. The RegEx class also contains some static methods: Escape: escape the escape characters in the RegEx string; Ismatch: If the expression matches a string, this method returns a Boolean value; Match: returns the instance of the match; Matches: returns a series of match methods; Replace: Replace the matching expression with the replacement string; Split: returns a series of strings determined by expressions; Unescape: do not escape characters in strings. Simple Matching First, we start to learn from simple expressions of the RegEx and match classes. Match m = RegEx. Match ("abracadabra", "(a | B | r) + "); We now have an instance of the match class that can be used for testing, for example: If (M. Success )... If you want to use a matched string, you can convert it into a string: Console. writeline ("match =" + M. tostring ()); In this example, the following output is obtained: match = Abra. This is the matched string. String replacement Simple string replacement is very intuitive. For example, the following statement: String S = RegEx. Replace ("Abracadabra", "Abra", "ZZZZ "); It returns the string zzzzzzcadzzzz, and all matched strings are replaced with zzzzzzz. Now let's look at a complicated string replacement example: String S = RegEx. Replace ("Abra", @ "^ \ s *(.*?) \ S * $ "," $1 "); This statement returns the string Abra, with leading and trailing spaces removed. The preceding mode is useful for deleting leading and trailing spaces in any string. In C #, we often use letter strings. In a letter string, compileProgram The character "\" is not treated as an escape character. When the character "\" is used to specify the escape character, @ "..." Is very useful. It is also worth mentioning that $1 is used in string replacement, which indicates that the replacement string can only contain the replaced string. Matching engine details Now, we use a group structure to understand a slightly complex example. See the following Example: String text = "Abracadabra1abracadabra2abracadabra3 "; String PAT = @" (# Start of the first group Abra # match the string Abra (# Start of the second group CAD # matching string CAD )? # End of the second group (optional) ) # End of the first group + # Match once or multiple times "; // Ignore comments using the x modifier RegEx r = new RegEx (Pat, "x "); // Obtain the group number list Int [] gnums = R. getgroupnumbers (); // Match for the first time Match m = R. Match (text ); While (M. Success) { // Start with Group 1 For (INT I = 1; I <gnums. length; I ++) { Group G = M. Group (gnums [I]); // Obtain the matched group Console. writeline ("group" + gnums [I] + "= [" + G. tostring () + "]"); // Calculate the start position and length of the Group Capturecollection cc = G. captures; For (Int J = 0; j <cc. Count; j ++) { Capture c = Cc [J]; Console. writeline ("capture" + J + "= [" + C. tostring () + "] Index =" + C. index + "length =" + C. Length ); } } // Next match M = M. nextmatch (); } The output of this example is as follows: Group1 = [Abra] Capture0 = [abracad] Index = 0 length = 7 Capture1 = [Abra] Index = 7 length = 4 Group2 = [CAD] Capture0 = [CAD] Index = 4 length = 3 Group1 = [Abra] Capture0 = [abracad] Index = 12 length = 7 Capture1 = [Abra] Index = 19 length = 4 Group2 = [CAD] Capture0 = [CAD] Index = 16 length = 3 Group1 = [Abra] Capture0 = [abracad] Index = 24 length = 7 Capture1 = [Abra] Index = 31 length = 4 Group2 = [CAD] Capture0 = [CAD] Index = 28 length = 3 First, we start by examining the string Pat, which contains an expression. The first capture starts with the first parentheses, and then the expression matches with an Abra. The second capture group starts from the second parentheses, but the first capture group is not over yet. This means that the first group matches abracad, the matching result of the second group is only CAD. Therefore, if you use? Symbol to make CAD an optional match, matching The result may be Abra or abracad. Then, the first group ends and the expression is required to be matched multiple times by specifying the + symbol. Now let's take a look at what happens in the matching process. First, call the constructor method of RegEx to create an instance of the expression and specify various options. In this example, because there is a comment in the expression, the X option is selected, and some spaces are used. When the X option is enabled, the expression ignores comments and spaces without escape. Then, retrieve the list of group numbers defined in the expression. Of course you can use these numbers explicitly. Here you use the programming method. If a named group is used, this method is also very effective as a way to create a fast index. The next step is to complete the first matching. Use a loop to test whether the current matching is successful. Next, repeat this operation on the group list from Group 1. In this example, group 0 is not used because group 0 is a fully matched string. To collect all matched strings as a single string, group 0 is used. We track the capturecollection in each group. Normally, each group can have only one capture, but group1 in this example has two capture: capture0 and capture1. If you only need group1 Tostring, only Abra will be obtained, and of course it will also match abracad. The value of tostring in the group is the value of the last capture in its capturecollection, which is exactly what we need. If you want the entire process to end after matching Abra, you should delete the + symbol from the expression to let the RegEx engine know that we only need to match the expression. Comparison between process-based and expression-based methods Generally, users who use rule expressions can be divided into the following two categories: the first type of users should try not to use rule expressions, but use procedures to perform operations that need to be repeated; the second type of users make full use of the functions and power of the Rule Expression Processing Engine, and use the process as little as possible. For most of our users, the best solution is to use both of them. I hope this article will illustrate the role of the Regexp class in the. NET language and its advantages and disadvantages between performance and complexity. Process-based model We often need to use a function in programming to match a part of a string or process other strings. Below is an example of matching words in a string: String text = "the quick red fox jumped over the lazy brown dog ."; System. Console. writeline ("text = [" + TEXT + "]"); String result = ""; String Pattern = @ "\ W + | \ W + "; Foreach (Match m in RegEx. Matches (text, pattern )) { // Obtain the matched string String x = M. tostring (); // If the first character is lowercase If (char. islower (X [0]) // Convert to uppercase X = Char. toupper (X [0]) + X. substring (1, x. Length-1 ); // Collect all characters Result + = X; } System. Console. writeline ("result = [" + Result + "]"); As shown in the preceding example, we use the foreach statement in C # to process each matching character and complete corresponding processing. In this example, a new result string is created. The output of this example is as follows: TEXT = [the quick red fox jumped over the lazy brown dog.] Result = [the quick red fox jumped over the lazy brown dog.] Expression-based mode Another way to complete the functions in the above example is through a matchevaluator, the new Code As follows: Static string captext (Match m) { // Obtain the matched string String x = M. tostring (); // If the first character is lowercase If (char. islower (X [0]) // Convert to uppercase Return Char. toupper (X [0]) + X. substring (1, x. Length-1 ); Return X; } Static void main () { String text = "the quick red fox jumped over Lazy brown dog ."; System. Console. writeline ("text = [" + TEXT + "]"); String Pattern = @ "\ W + "; String result = RegEx. Replace (text, pattern, New matchevaluator (test. captext )); System. Console. writeline ("result = [" + Result + "]"); } At the same time, it is important to note that this mode is very simple because you only need to modify words without modifying non-words. Common expressions To better understand how to use rule expressions in the C # environment, I have written some rule expressions that may be useful to you. These expressions have been used in other environments, hope to help you. 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 ); Exchange the first two words String t2 = "The quick brown fox "; String P2 = @ "(\ s + )"; RegEx X2 = new RegEx (P2 ); String r2 = x2.replace (t2, "$3 $2 $1", 1 ); Guan jianzi = Value String T3 = "myval = 3 "; String P3 = @ "(\ W +) \ s * = \ s * (. *) \ s * $ "; Match m3 = RegEx. Match (T3, P3 ); Implement 80 characters per line String t4 = "********************" + "******************************" + "******************************"; String P4 = ". {80 ,}"; Match M4 = RegEx. Match (T4, P4 ); Month/day/year hour: minute: Second Time Format String T5 = "01/01/01 16:10:01 "; String P5 = @ "(\ D +)/(\ D +) :( \ D + )"; Match M5 = RegEx. Match (T5, P5 ); Change directory (applicable only to Windows) String T6 = @ "C: \ Documents ents and Settings \ user1 \ Desktop \"; String R6 = RegEx. Replace (T6, @ "\ user1 \", @ "\ user2 \\"); Extended 16-bit escape characters String T7 = "% 41"; // capital String P7 = "% ([0-9a-fa-f] [0-9a-fa-f])"; String R7 = RegEx. Replace (T7, P7, hexconvert ); Delete comments in C Language (to be improved) String T8 = @" /* * Notes for traditional styles */ "; String P8 = @" /\ * # Match the delimiters starting with the Annotation .*? # Matching comments \ */# End Separator of matching Annotation "; String R8 = RegEx. Replace (T8, p8, "", "XS "); Deletes spaces at the start and end of a string. String t9a = "leading "; String p9a = @ "^ \ s + "; String r9a = RegEx. Replace (t9a, p9a ,""); String t9b = "trailing "; String p9b = @ "\ s + $ "; String r9b = RegEx. Replace (t9b, p9b ,""); Add character N after character \ to make it a real New Line String t10 = @ "\ ntest \ n "; String R10 = RegEx. Replace (T10, @ "\ n", "\ n "); Convert IP addresses String T11 = "55.54.53.52 "; String P11 = "^" + @ "([01]? \ D \ d | 2 [0-4] \ d | 25 [0-5]) \. "+ @ "([01]? \ D \ d | 2 [0-4] \ d | 25 [0-5]) \. "+ @ "([01]? \ D \ d | 2 [0-4] \ d | 25 [0-5]) \. "+ @ "([01]? \ D \ d | 2 [0-4] \ d | 25 [0-5]) "+ "$ "; Match M11 = RegEx. Match (T11, P11 ); Delete the path contained in the file name String T12 = @ "C: \ file.txt "; String p12 = @ "^ .*\\"; String R12 = RegEx. Replace (T12, p12 ,""); Concatenate rows from multiple strings String t13 = @ "this is A split line "; String P13 = @ "\ s * \ r? \ N \ s *"; String R13 = RegEx. Replace (t13, P13 ,""); Extract all numbers in the string String T14 = @" Test 1 Test 1, 2.3 Test 47 "; String p14 = @ "(\ D + \.? \ D * | \. \ D + )"; Matchcollection mc14 = RegEx. Matches (T14, p14 ); Find all uppercase letters String T15 = "this is a test of all caps "; String P15 = @ "(\ B [^ \ Wa-z0-9 _] + \ B )"; Matchcollection mc15 = RegEx. Matches (T15, p15 ); Find lowercase words String T16 = "this is a test of lowercase "; String p16 = @ "(\ B [^ \ WA-Z0-9 _] + \ B )"; Matchcollection mc16 = RegEx. Matches (T16, p16 ); Find the words whose first letter is uppercase. String t17 = "this is a test of initial caps "; String P17 = @ "(\ B [^ \ Wa-z0-9 _] [^ \ WA-Z0-9 _] * \ B )"; Matchcollection mc17 = RegEx. Matches (t17, P17 ); Find links in a simple HTML language String T18 = @" <HTML> <A href = "" first.htm ""> first tag text </a> <A href = "" next.htm ""> next tag text </a> </Html> "; String P18 = @ "<A [^>] *? Href \ s * = \ s * ["" ']? "+ @" ([^ '">] + ?) ['""]?> "; Matchcollection mc18 = RegEx. Matches (T18, P18, "Si ");