Most efficient way to remove special characters from string

Question

I want to remove all special characters from a string  Allowed characters are A-Z  uppercase or lowercase   numbers  0-9   underscore      or the dot sign       I have the following  it works but I suspect  I know   it s not very efficient       public static string RemoveSpecialCharacters string str                StringBuilder sb   new StringBuilder            for  int i   0  i  lt  str Length  i                          if   str i   gt    0   amp  amp  str i   lt    9                       str i   gt    A   amp  amp  str i   lt    z                          str i            str i                                                  sb Append str i                                         return sb ToString            What is the most efficient way to do this  What would a regular expression look like  and how does it compare with normal string manipulation   The strings that will be cleaned will be rather short  usually between 10 and 30 characters in length

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public static string RemoveSpecialCharacters string str        char   buffer   new char str Length       int idx   0       foreach  char c in str                if   c  gt    0   amp  amp  c  lt    9       c  gt    A   amp  amp  c  lt    Z                   c  gt    a   amp  amp  c  lt    z       c             c                                buffer idx    c              idx                         return new string buffer  0  idx

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Why do you think that your method is not efficient  It s actually one of the most efficient ways that you can do it   You should of course read the character into a local variable or use an enumerator to reduce the number of array accesses   public static string RemoveSpecialCharacters this string str       StringBuilder sb   new StringBuilder       foreach  char c in str          if   c  gt    0   amp  amp  c  lt    9       c  gt    A   amp  amp  c  lt    Z       c  gt    a   amp  amp  c  lt    z      c           c                    sb Append c                   return sb ToString        One thing that makes a method like this efficient is that it scales well  The execution time will be relative to the length of the string  There is no nasty surprises if you would use it on a large string   Edit  I made a quick performance test  running each function a million times with a 24 character string  These are the results   Original function  54 5 ms  My suggested change  47 1 ms  Mine with setting StringBuilder capacity  43 3 ms  Regular expression  294 4 ms   Edit 2  I added the distinction between A-Z and a-z in the code above   I reran the performance test  and there is no noticable difference    Edit 3  I tested the lookup char   solution  and it runs in about 13 ms   The price to pay is  of course  the initialization of the huge lookup table and keeping it in memory  Well  it s not that much data  but it s much for such a trivial function     private static bool    lookup   static Program         lookup   new bool 65536      for  char c    0   c  lt    9   c     lookup c    true     for  char c    A   c  lt    Z   c     lookup c    true     for  char c    a   c  lt    z   c     lookup c    true      lookup        true      lookup        true     public static string RemoveSpecialCharacters string str       char   buffer   new char str Length      int index   0     foreach  char c in str          if   lookup c              buffer index    c           index                    return new string buffer  0  index

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There are lots of proposed solutions here  some more efficient than others  but perhaps not very readable   Here s one that may not be the most efficient  but certainly usable for most situations  and is quite concise and readable  leveraging Linq   string stringToclean    This is a test   Do not try this at home  you might get hurt  Don t believe it     var validPunctuation   new HashSet lt char gt     -     var cleanedVersion   new String stringToclean Where x   gt   x  gt    A   amp  amp  x  lt    Z       x  gt    a   amp  amp  x  lt    z      validPunctuation Contains x   ToArray      var cleanedLowercaseVersion   new String stringToclean ToLower   Where x   gt   x  gt    a   amp  amp  x  lt    z      validPunctuation Contains x   ToArray

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It seems good to me  The only improvement I would make is to initialize the StringBuilder with the length of the string   StringBuilder sb   new StringBuilder str Length

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Use   s erase std  remove if s begin    s end    my predicate   s end      bool my predicate char c     return   isalpha c     c         c           depending on you definition of special characters     And you ll get a clean string s   erase   will strip it of all the special characters and is highly customisable with the my predicate   function

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I agree with this code sample  The only different it I make it into Extension Method of string type  So that you can use it in a very simple line or code   string test    abc   123   test RemoveSpecialCharacters      Thank to Guffa for your experiment   public static class MethodExtensionHelper           public static string RemoveSpecialCharacters this string str                        StringBuilder sb   new StringBuilder                foreach  char c in str                                if   c  gt    0   amp  amp  c  lt    9       c  gt    A   amp  amp  c  lt    Z       c  gt    a   amp  amp  c  lt    z      c                                               sb Append c                                               return sb ToString

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public string RemoveSpecial string evalstr    StringBuilder finalstr   new StringBuilder                foreach char c in evalstr               int charassci   Convert ToInt16 c               if    charassci  gt   33  amp  amp  charassci  lt   47     special char                  finalstr append c                 return finalstr ToString

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A regular expression will look like   public string RemoveSpecialChars string input        return Regex Replace input      0-9a-zA-Z       string Empty       But if performance is highly important  I recommend you to do some benchmarks before selecting the  regex path

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HashSet is O 1  Not sure if it is faster than the existing comparison    private static HashSet lt char gt  ValidChars   new HashSet lt char gt       a    b    c    A    B    C    1    2    3          public static string RemoveSpecialCharacters string str        StringBuilder sb   new StringBuilder str Length   2       foreach  char c in str                if  ValidChars Contains c   sb Append c             return sb ToString        I tested and this in not faster than the accepted answer  I will leave it up as if you needed a configurable set of characters this would be a good solution

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If you re worried about speed  use pointers to edit the existing string  You could pin the string and get a pointer to it  then run a for loop over each character  overwriting each invalid character with a replacement character   It would be extremely efficient and would not require allocating any new string memory   You would also need to compile your module with the unsafe option  and add the  unsafe  modifier to your method header in order to use pointers   static void Main string   args        string str    string   with  amp  invalid  characters       Console WriteLine  str      print original string     FixMyString  str             Console WriteLine  str      print string again to verify that it has been modified     Console ReadLine      pause to leave command prompt open     public static unsafe void FixMyString  string str  char replacement char         fixed  char  p str   str                char  c   p str    temp pointer  since p str is read-only         for  int i   0  i  lt  str Length  i    c      loop through each character in string  advancing the character pointer as well             if   IsValidChar  c     check whether the current character is invalid                   c    replacement char    overwrite character in existing string with replacement character          public static bool IsValidChar  char c         return  c  gt    0   amp  amp  c  lt    9       c  gt    A   amp  amp  c  lt    Z       c  gt    a   amp  amp  c  lt    z       c           c                return char IsLetterOrDigit  c      c           c           this may work as well

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public static string RemoveSpecialCharacters string str       return str replaceAll    A-Za-z0-9

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For S amp G s  Linq-ified way   var original        foo     amp     gt  lt  gt         -     var valid   new char           a    b    c    d    e    f    g    h    i    j    k    l    m    n    o         p    q    r    s    t    u    v    w    x    y    z    A    B    C    D         E    F    G    H    I    J    K    L    M    N    O    P    Q    R    S         T    U    V    W    X    Y    Z    1    2    3    4    5    6    7    8         9    0               var result   string Join          from x in original ToCharArray         where valid Contains x  select x ToString             ToArray       I don t think this is going to be the most efficient way  however

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StringBuilder sb   new StringBuilder     for  int i   0  i  lt  fName Length  i         if  char IsLetterOrDigit fName i                 sb Append fName i

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I had to do something similar for work  but in my case I had to filter all that is not a letter  number or whitespace  but you could easily modify it to your needs   The filtering is done client-side in JavaScript  but for security reasons I am also doing the filtering server-side  Since I can expect most of the strings to be clean  I would like to avoid copying the string unless I really need to  This let my to the implementation below  which should perform better for both clean and dirty strings   public static string EnsureOnlyLetterDigitOrWhiteSpace string input        StringBuilder cleanedInput   null      for  var i   0  i  lt  input Length    i                var currentChar   input i           var charIsValid   char IsLetterOrDigit currentChar     char IsWhiteSpace currentChar            if  charIsValid                        if cleanedInput    null                  cleanedInput Append currentChar                     else                       if  cleanedInput    null  continue              cleanedInput   new StringBuilder                if  i  gt  0                  cleanedInput Append input Substring 0  i                         return cleanedInput    null   input   cleanedInput ToString

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You can use regular expresion as follows   return Regex Replace strIn       w   -        RegexOptions None  TimeSpan FromSeconds 1 0

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If you re using a dynamic list of characters  LINQ may offer a much faster and graceful solution   public static string RemoveSpecialCharacters string value  char   specialCharacters        return new String value Except specialCharacters  ToArray         I compared this approach against two of the previous  fast  approaches  release compilation     Char array solution by LukeH - 427 ms StringBuilder solution - 429 ms LINQ  this answer  - 98 ms   Note that the algorithm is slightly modified - the characters are passed in as an array rather than hard-coded  which could be impacting things slightly  ie  the other solutions would have an inner foor loop to check the character array    If I switch to a hard-coded solution using a LINQ where clause  the results are    Char array solution - 7ms StringBuilder solution - 22ms LINQ - 60 ms   Might be worth looking at LINQ or a modified approach if you re planning on writing a more generic solution  rather than hard-coding the list of characters  LINQ definitely gives you concise  highly readable code - even more so than Regex

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I wonder if a Regex-based replacement  possibly compiled  is faster  Would have to test that Someone has found this to be  5 times slower   Other than that  you should initialize the StringBuilder with an expected length  so that the intermediate string doesn t have to be copied around while it grows   A good number is the length of the original string  or something slightly lower  depending on the nature of the functions inputs    Finally  you can use a lookup table  in the range 0  127  to find out whether a character is to be accepted

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I suggest creating a simple lookup table  which you can initialize in the static constructor to set any combination of characters to valid   This lets you do a quick  single check   edit  Also  for speed  you ll want to initialize the capacity of your StringBuilder to the length of your input string   This will avoid reallocations   These two methods together will give you both speed and flexibility   another edit  I think the compiler might optimize it out  but as a matter of style as well as efficiency  I recommend foreach instead of for

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Well  unless you really need to squeeze the performance out of your function  just go with what is easiest to maintain and understand  A regular expression would look like this   For additional performance  you can either pre-compile it or just tell it to compile on first call  subsequent calls will be faster    public static string RemoveSpecialCharacters string str        return Regex Replace str     a-zA-Z0-9           RegexOptions Compiled

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public static string RemoveAllSpecialCharacters this string text      if  string IsNullOrEmpty text       return text     string result   Regex Replace text            amp            gt  lt                           return result

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The following code has the following output  conclusion is that we can also save some memory resources allocating array smaller size    lookup   new bool 123    for  var c    0   c  lt    9   c          lookup c    true  System Diagnostics Debug WriteLine  int c           char c      for  var c    A   c  lt    Z   c          lookup c    true  System Diagnostics Debug WriteLine  int c           char c      for  var c    a   c  lt    z   c          lookup c    true  System Diagnostics Debug WriteLine  int c           char c      48  0   49  1   50  2   51  3   52  4   53  5   54  6   55  7   56  8   57  9   65  A   66  B   67  C   68  D   69  E   70  F   71  G   72  H   73  I   74  J   75  K   76  L   77  M   78  N   79  O   80  P   81  Q   82  R   83  S   84  T   85  U   86  V   87  W   88  X   89  Y   90  Z   97  a   98  b   99  c   100  d   101  e   102  f   103  g   104  h   105  i   106  j   107  k   108  l   109  m   110  n   111  o   112  p   113  q   114  r   115  s   116  t   117  u   118  v   119  w   120  x   121  y   122  z     You can also add the following code lines to support Russian locale  array size will be 1104    for  var c        c  lt        c          lookup c    true  System Diagnostics Debug WriteLine  int c           char c      for  var c        c  lt        c          lookup c    true  System Diagnostics Debug WriteLine  int c           char c

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I m not sure it is the most efficient way  but It works for me   Public Function RemoverTildes stIn As String  As String     Dim stFormD As String   stIn Normalize NormalizationForm FormD      Dim sb As New StringBuilder        For ich As Integer   0 To stFormD Length - 1         Dim uc As UnicodeCategory   CharUnicodeInfo GetUnicodeCategory stFormD ich           If uc  lt  gt  UnicodeCategory NonSpacingMark Then             sb Append stFormD ich           End If     Next     Return  sb ToString   Normalize NormalizationForm FormC   End Function

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I m not convinced your algorithm is anything but efficient  It s O n  and only looks at each character once  You re not gonna get any better than that unless you magically know values before checking them   I would however initialize the capacity of your StringBuilder to the initial size of the string  I m guessing your perceived performance problem comes from memory reallocation   Side note  Checking A-z is not safe  You re including                and       Side note 2  For that extra bit of efficiency  put the comparisons in an order to minimize the number of comparisons   At worst  you re talking 8 comparisons tho  so don t think too hard   This changes with your expected input  but one example could be   if  str i   gt    0   amp  amp  str i   lt    z   amp  amp        str i   gt    a     str i   lt    9       str i   gt    A   amp  amp  str i   lt    Z           str i             str i            Side note 3  If for whatever reason you REALLY need this to be fast  a switch statement may be faster  The compiler should create a jump table for you  resulting in only a single comparison   switch  str i         case  0       case  1                         case              sb Append str i            break

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I would use a String Replace with a Regular Expression searching for  special characters   replacing all characters found with an empty string

[c#] Most efficient way to remove special characters from string

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