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Thread: using Regular Expression for simple searching

  1. 14th September 2011, 14:46 #1
    sparticus_37
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    Default using Regular Expression for simple searching

    I was recently given an assignment for a C++ class im currently taking to search the given string

    BRSLPMSLPRESLPSLPSLPABSLPENDRPCFPSLPABXDSLPRMNPLSL RSLPBESLPASLPRTWPSLPXBTSLPSLPM

    using a character array approach, for occurrences of the substring SLP and output the number of times SLP occurs. Error conditions for searching the string are:
    1. SLP cannot be preceded by E. ex.) ESLP
    2. SLP cannot be followed by A. ex.)SLPA
    3. SLP cannot be preceded by B followed by any character. ex.) BASLP , BBSLP ……., BZSLP

    But using a char array approach is too simple and boring, for me the solution was
    Qt Code: 
    1. int count =0;
    2. QString data = "BRSLPMSLPRESLPSLPSLPABSLPENDRPCFPSLPABXDSLPRMNPLSLRSLPBESLPASLPRTWPSLPXBTSLPSLPM";
    3. for( int i =0; i < data.length(); ++i){
    4.  
    5.         if(data[i] == QChar('S') && data[i+1] == QChar('L') && data[i+2] == QChar('P')){
    6.             if(data[i-2] != QChar('B') && data[i-1] != QChar('E') &&data[i+3] != QChar('A'))
    7.                 ++count;
    8.         }
    9.     }
    10. // count = 8
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    so i decided to see if i could accomplish the same thing using regular expressions, which i just recently started learning on my own time.


    Qt Code: 
    1. #include <iostream>
    2. #include <QRegExp>
    3. #include <QString>
    4. int 
    5. int main(){
    6. 	QString data = "BRSLPMSLPRESLPSLPSLPABSLPENDRPCFPSLPABXDSLPRMNPLSLRSLPBESLPASLPRTWPSLPXBTSLPSLPM"
    7. 	QRegExp regxTotal("SLP+"); // match string "SLP" - output is 14
    8. 	QRegExp aCount("SLPA+"); // match string "SLPA" -  output is  3
    9. 	QRegExp eCount("ESLP+"); // match string "ESLP" output  is 2
    10. 	QRegExp bCount("B.SLP+"); // match string beginning with B followed by any character followed by SLP - output is 3
    11. 	QRegExp woErrors("([^E]SLP[^A])"); // match  SLP that do not meet error conditions 
    12. 	/*
    13.  
    14. 	Attempted variations
    15.         		((?!E)SLP+|(?!B.)SLP+|SLP(?!A)+)    output is 14
    16.         		((?!ESLP)+|(?!B.SLP)+|(?!ASLP)+)   no output, causes infinite loop
    17.         		((?!ESLP)SLP+|(?!B.SLP)SLP+|(?!ASLP)SLP+)  output is 14
    18. 	*/
    19.  
    20.  
    21. 	std:: cout << "" << courtOccurances(regxTotal, data) << std::endl; // output is 14
    22. 	std:: cout << "" << courtOccurances(aCount, data) << std::endl; // output is 3
    23. 	std:: cout << "" << courtOccurances(eCount, data) << std::endl; // output is 2
    24. 	std:: cout << "" << courtOccurances(bCount, data) << std::endl; // output is 3
    25. 	std:: cout << "" << courtOccurances(woErrors, data,true) << std::endl; // output should be 8
    26.  
    27.  
    28. 	return 0;
    29.  
    30. }//end main
    31.  
    32.  
    33.  
    34. int countOccurances(QRegExp & regx, QString& data, bool debug = false){
    35. 	int count = 0; // occurances
    36. 	int pos = 0; // current pos inside of 'data'
    37.  	while ((pos = regx.indexIn(data, pos)) != -1) {
    38. 		++count;
    39.          		pos += regx.matchedLength();
    40.  
    41.          		if(debug){ // print captured texts to console for debug
    42.             		QStringList tmp = regx.capturedTexts();
    43.            			 while(! tmp.isEmpty()){
    44. 				std::cout << tmp.first().toStdString() << std::endl;	
    45. 				tmp.removeFirst();
    46. 			}// end while
    47.         		 }//end if
    48.      }// end while
    49.      if(debug)// print debug output seperator
    50.             std::cout << "+++++++++++++++++"<< data.toStdString() <<std::endl;
    51.  
    52.  
    53.     return count;
    54. }// end fucntion
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    Output of captured texts
    Qt Code: 
    1. ((?!E)SLP+|(?!B.)SLP+|SLP(?!A)+)  
    2. 	SLP
    3. SLP
    4. SLP
    5. SLP
    6. SLP
    7. SLP
    8. SLP
    9. SLP
    10. SLP
    11. SLP
    12. SLP
    13. SLP
    14. SLP
    15. SLP
    16. SLP
    17. SLP
    18. SLP
    19. SLP
    20. SLP
    21. SLP
    22. SLP
    23. SLP
    24. SLP
    25. SLP
    26. SLP
    27. SLP
    28. SLP
    29. SLP
    30.  
    31. ((?!ESLP)+|(?!B.SLP)+|(?!SLPA)+) 
    32. 	expression unmatched- somehow causes infinite loop 
    33.  
    34. ((?!ESLP)SLP+|(?!B.SLP)SLP+|(?!ASLP)SLP+) count is 14
    35.  
    36. SLP
    37. SLP
    38. SLP
    39. SLP
    40. SLP
    41. SLP
    42. SLP
    43. SLP
    44. SLP
    45. SLP
    46. SLP
    47. SLP
    48. SLP
    49. SLP
    50. SLP
    51. SLP
    52. SLP
    53. SLP
    54. SLP
    55. SLP
    56. SLP
    57. SLP
    58. SLP
    59. SLP
    60. SLP
    61. SLP
    62. SLP
    63. SLP
    To copy to clipboard, switch view to plain text mode 


    any tips for how I can get QRegExp to meet my error conditions? if any more info is needed plz ask. thanks for the help.
    artificial intelligence is no match for natural stupidity
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  2. 14th September 2011, 15:10 #2
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    wysota
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    Default Re: using Regular Expression for simple searching

    Your search approach is a bit naive. If your array has n characters, you have to do n iterations over the array. That's too much, it is possible to do the search more efficiently (without using regular expressions).
    Your biological and technological distinctiveness will be added to our own. Resistance is futile.

    Please ask Qt related questions on the forum and not using private messages or visitor messages.

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  3. 14th September 2011, 16:37 #3
    sparticus_37
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    Default Re: using Regular Expression for simple searching

    it is possible to do the search more efficiently (without using regular expressions).
    I am aware that there are more efficient ways to approach searching a string( most of which i don't know how to implement ) , i just wanted to try i using regular expressions to see if i could get the same results. the program itself is for a class assignment so i am not really concerned about efficiency, just getting the correct result.
    artificial intelligence is no match for natural stupidity
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