What function is to replace a substring from a string in C

Question

Given a  char    string  I want to find all occurrences of a substring and replace them with an alternate string  I do not see any simple function that achieves this in  lt string h gt

User · Answer

i find most of the proposed functions hard to understand - so i came up with this:

static char *dull_replace(const char *in, const char *pattern, const char *by)
{
    size_t outsize = strlen(in) + 1;
    // TODO maybe avoid reallocing by counting the non-overlapping occurences of pattern
    char *res = malloc(outsize);
    // use this to iterate over the output
    size_t resoffset = 0;

    char *needle;
    while (needle = strstr(in, pattern)) {
        // copy everything up to the pattern
        memcpy(res + resoffset, in, needle - in);
        resoffset += needle - in;

        // skip the pattern in the input-string
        in = needle + strlen(pattern);

        // adjust space for replacement
        outsize = outsize - strlen(pattern) + strlen(by);
        res = realloc(res, outsize);

        // copy the pattern
        memcpy(res + resoffset, by, strlen(by));
        resoffset += strlen(by);
    }

    // copy the remaining input
    strcpy(res + resoffset, in);

    return res;
}

output must be free'd

User · Answer

Here is the one that I created based on these requirements    Replace the pattern regardless of whether is was long or shorter  Not use any malloc  explicit or implicit  to intrinsically avoid memory leaks  Replace any number of occurrences of pattern  Tolerate the replace string having a substring equal to the search string  Does not have to check that the Line array is sufficient in size to hold the replacement   e g  This does not work unless the caller knows that line is of sufficient size to hold the new string         returns number of strings replaced     int replacestr char  line  const char  search  const char  replace       int count     char  sp     start of pattern       printf  replacestr  s   s   s  n   line  search  replace      if   sp   strstr line  search      NULL          return 0           count   1     int sLen   strlen search      int rLen   strlen replace      if  sLen  gt  rLen             move from right to left       char  src   sp   sLen        char  dst   sp   rLen        while   dst    src       0     dst    src           else if  sLen  lt  rLen             move from left to right       int tLen   strlen sp  - sLen        char  stop   sp   rLen        char  src   sp   sLen   tLen        char  dst   sp   rLen   tLen        while dst  gt   stop     dst    src  dst--  src--            memcpy sp  replace  rLen       count    replacestr sp   rLen  search  replace       return count       Any suggestions for improving this code are cheerfully accepted   Just post the comment and I will test it

User · Answer

a fix to fann95 s response  using in-place modification of the string  and assuming the buffer pointed to by line is large enough to hold the resulting string   static void replacestr char  line  const char  search  const char  replace         char  sp        if   sp   strstr line  search      NULL             return              int search len   strlen search        int replace len   strlen replace        int tail len   strlen sp search len         memmove sp replace len sp search len tail len 1        memcpy sp  replace  replace len

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Using only strlen from string h  sorry for my English  char   str replace char   text char   rep  char   repw    text - gt  to replace in it   rep - gt  replace   repw - gt  replace with     int replen   strlen rep  repwlen   strlen repw  count   some constant variables     for int i 0 i lt strlen text  i      search for the first character from rep in text         if text i     rep 0     if it found it             count   1   start searching from the next character to avoid repetition             for int j 1 j lt replen j                     if text i j     rep j     see if the next character in text is the same as the next in the rep if not break                     count                     else                      break                                              if count    replen    if count equals to the lenght of the rep then we found the word that we want to replace in the text                 if replen  lt  repwlen                       for int l   strlen text  l gt i l--    cuz repwlen greater than replen we need to shift characters to the right to make space for the replacement to fit                         text l repwlen-replen    text l    shift by repwlen-replen                                                         if replen  gt  repwlen                       for int l i replen-repwlen l lt strlen text  l      cuz replen greater than repwlen we need to shift the characters to the left                         text l- replen-repwlen     text l    shift by replen-repwlen                                           text strlen text - replen-repwlen       0    get rid of the last unwanted characters                                   for int l 0 l lt repwlen l      replace rep with repwlen                     text i l    repw l                                     if replen    repwlen                       i  repwlen-1   pass to the next character   try text  y   rep  y  repw  yy  without this line to understand                                                     return text      if you want strlen code to avoid calling string h  int strlen char   string    use this code to avoid calling string h     int lenght   0      while string lenght       0            lenght              return lenght

User · Answer

Here is the code for unicode strings    int mystrstr wchar t  txt1 wchar t  txt2        wchar t  posstr wcsstr txt1 txt2       if posstr  NULL                return  posstr-txt1        else               return -1              assume  supplied buff is enough to hold generated text void StringReplace wchar t  buff wchar t  txt1 wchar t  txt2        wchar t  tmp      wchar t  nextStr      int pos       tmp wcsdup buff        pos mystrstr tmp txt1       if pos  -1                buff 0  0          wcsncpy buff tmp pos           buff pos  0           wcscat buff txt2            nextStr tmp pos wcslen txt1            while wcslen nextStr   0                        pos mystrstr nextStr txt1                if pos  -1                                wcscat buff nextStr                   break                             wcsncat buff nextStr pos               wcscat buff txt2                nextStr nextStr pos wcslen txt1                           free tmp

User · Answer

char  replace const char instring  const char  old part  const char  new part      ifndef EXPECTED REPLACEMENTS      define EXPECTED REPLACEMENTS 100  endif      if  instring     old part     new part                return  char  NULL             size t instring len strlen instring       size t new len strlen new part       size t old len strlen old part       if instring len lt old len    old len  0                return  char  NULL             const char  in instring      const char  found NULL      size t count 0      size t out 0      size t ax 0      char  outstring NULL       if new len gt  old len                 size t Diff EXPECTED REPLACEMENTS  new len-old len           size t outstring len instring len   Diff          outstring   char   malloc outstring len            if  outstring               return  char  NULL                    while  found   strstr in  old part    NULL                        if count  EXPECTED REPLACEMENTS                                outstring len  Diff                  if  outstring realloc outstring outstring len    NULL                                         return  char  NULL                                    count 0                            ax found-in              strncpy outstring out in ax               out  ax              strncpy outstring out new part new len               out  new len              in found old len              count                        else               outstring   char   malloc instring len           if  outstring               return  char  NULL                    while  found   strstr in  old part    NULL                        ax found-in              strncpy outstring out in ax               out  ax              strncpy outstring out new part new len               out  new len              in found old len                      ax  instring instring len -in      strncpy outstring out in ax       out  ax      outstring out    0        return outstring

User · Answer

The repl str   function on creativeandcritical net is fast and reliable  Also included on that page is a wide string variant  repl wcs    which can be used with Unicode strings including those encoded in UTF-8  through helper functions - demo code is linked from the page  Belated full disclosure  I am the author of that page and the functions on it

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You could build your own replace function using strstr to find the substrings and strncpy to copy in parts to a new buffer     Unless what you want to replace with is the same length as what you you want to replace  then it s probably best to use a new buffer to copy the new string to

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DWORD ReplaceString   inout PCHAR source    in DWORD dwSourceLen    in const char  pszTextToReplace    in const char  pszReplaceWith        DWORD dwRC   NO ERROR      PCHAR foundSeq   NULL      PCHAR restOfString   NULL      PCHAR searchStart   source      size t szReplStrcLen   strlen pszReplaceWith   szRestOfStringLen   0  sztextToReplaceLen   strlen pszTextToReplace   remainingSpace   0  dwSpaceRequired   0      if  strcmp pszTextToReplace         0          dwRC   ERROR INVALID PARAMETER      else if  strcmp pszTextToReplace  pszReplaceWith     0                do                       foundSeq   strstr searchStart  pszTextToReplace               if  foundSeq                                szRestOfStringLen    strlen foundSeq  - sztextToReplaceLen    1                  remainingSpace   dwSourceLen -  foundSeq - source                   dwSpaceRequired   szReplStrcLen    szRestOfStringLen                   if  dwSpaceRequired  gt  remainingSpace                                        dwRC   ERROR MORE DATA                                     else                                       restOfString   CMNUTIL calloc szRestOfStringLen  sizeof CHAR                        strcpy s restOfString  szRestOfStringLen  foundSeq   sztextToReplaceLen                        strcpy s foundSeq  remainingSpace  pszReplaceWith                       strcat s foundSeq  remainingSpace  restOfString                                      CMNUTIL free restOfString                   searchStart   foundSeq   szReplStrcLen    search in the remaining str   avoid loops when replWith contains textToRepl                          while  foundSeq  amp  amp  dwRC    NO ERROR             return dwRC

User · Answer

As strings in C can not dynamically grow inplace substitution will generally not work  Therefore you need to allocate space for a new string that has enough room for your substitution and then copy the parts from the original plus the substitution into the new string  To copy the parts you would use strncpy

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This is not provided in the standard C library because  given only a char  you can t increase the memory allocated to the string if the replacement string is longer than the string being replaced   You can do this using std  string more easily  but even there  no single function will do it for you

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There isn t one   You d need to roll your own using something like strstr and strcat or strcpy

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Here goes mine  it s self contained and versatile  as well as efficient  it grows or shrinks buffers as needed in each recursion  void strreplace char  src  char  str  char  rep        char  p   strstr src  str       if  p                int len   strlen src  strlen rep -strlen str           char r len           memset r  0  len           if   p  gt   src                strncpy r  src  p-src               r p-src    0               strncat r  rep  strlen rep                strncat r  p strlen str   p strlen str -src strlen src                strcpy src  r               strreplace p strlen rep   str  rep

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There you go    this is the function to replace every occurance of char x with char y within character string str  char  zStrrep char  str  char x  char y       char  tmp str      while  tmp          if  tmp    x               tmp     y     assign first  then incement            else              tmp          tmp   0       return str      An example usage could be    Exmaple Usage         char s    this is a trial string to test the function            char x      y              printf   s n  zStrrep s x y       Example Output         this is a trial string to test the function    The function is from a string library I maintain on Github  you are more than welcome to have a look at other available functions or even contribute to the code     https   github com fnoyanisi zString  EDIT   siride is right  the function above replaces chars only  Just wrote this one  which replaces character strings    include  lt stdio h gt   include  lt stdlib h gt      replace every occurance of string x with string y    char  zstring replace str char  str  const char  x  const char  y       char  tmp str   str   tmp x   x   dummy ptr   tmp x   tmp y   y      int len str 0  len y 0  len x 0          string length        for    tmp y    len y    tmp y                 for    tmp str    len str    tmp str                 for    tmp x    len x    tmp x                    Bounds check        if  len y  gt   len str          return str          reset tmp pointers        tmp y   y      tmp x   x       for  tmp str   str    tmp str    tmp str          if  tmp str     tmp x                   save tmp str                for  dummy ptr tmp str   dummy ptr     tmp x    tmp x    dummy ptr                  if    tmp x 1       0   amp  amp    dummy ptr-str len y   lt  len str                       Reached end of x  we got something to replace then                    Copy y only if there is enough room for it                                        for tmp y y   tmp y    tmp y    tmp str                           tmp str    tmp y                           reset tmp x            tmp x   x                 return str     int main         char s    Free software is a matter of liberty  not price  n                To understand the concept  you should think of  free   n                as in  free speech   not as in  free beer         printf   s n n  s       printf   s n  zstring replace str s  ree   XYZ         return 0      And below is the output  Free software is a matter of liberty  not price  To understand the concept  you should think of  free   as in  free speech   not as in  free beer   FXYZ software is a matter of liberty  not price  To understand the concept  you should think of  fXYZ   as in  fXYZ speech   not as in  fXYZ beer

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You can use strrep    char  strrep        const char      cadena          const char      strf          const char      strr                strrep  String Replace   Replaces  strf  with  strr  in  cadena  and returns the new string  You need to free the returned string in your code after using strrep   Parameters     cadena  The string with the text      strf    The text to find      strr    The replacement text   Returns     The text updated wit the replacement    Project can be found at https   github com ipserc strrep

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This function only works if ur string has extra space for new length  void replace str char  str char  org char  rep        char  ToRep   strstr str org       char  Rest    char  malloc strlen ToRep        strcpy Rest   ToRep  strlen org          strcpy ToRep rep       strcat ToRep Rest        free Rest       This only replaces First occurrence

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char  replace char char  str  char in  char out        char   p   str       while p      0             if  p    in               p    out            p             return str

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You can use this function  the comments explain how it works    void strreplace char  string  const char  find  const char  replaceWith       if strstr string  replaceWith     NULL           char  temporaryString   malloc strlen strstr string  find    strlen find     1           strcpy temporaryString  strstr string  find    strlen find         Create a string with what s after the replaced part          strstr string  find      0        Take away the part to replace and the part after it in the initial string         strcat string  replaceWith        Concat the first part of the string with the part to replace with         strcat string  temporaryString        Concat the first part of the string with the part after the replaced part         free temporaryString        Free the memory to avoid memory leaks

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Here s some sample code that does it        include  lt string h gt   include  lt stdlib h gt   char   replace      char const   const original       char const   const pattern       char const   const replacement       size t const replen   strlen replacement     size t const patlen   strlen pattern     size t const orilen   strlen original      size t patcnt   0    const char   oriptr    const char   patloc        find how many times the pattern occurs in the original string   for  oriptr   original  patloc   strstr oriptr  pattern   oriptr   patloc   patlen          patcnt                    allocate memory for the new string     size t const retlen   orilen   patcnt    replen - patlen       char   const returned    char    malloc  sizeof char     retlen   1          if  returned    NULL                 copy the original string            replacing all the instances of the pattern       char   retptr   returned        for  oriptr   original  patloc   strstr oriptr  pattern   oriptr   patloc   patlen                  size t const skplen   patloc - oriptr             copy the section until the occurence of the pattern         strncpy retptr  oriptr  skplen           retptr    skplen             copy the replacement          strncpy retptr  replacement  replen           retptr    replen                   copy the rest of the string        strcpy retptr  oriptr             return returned          include  lt stdio h gt  int main int argc  char   argv        if  argc    4          fprintf stderr  usage   s  lt original text gt   lt pattern gt   lt replacement gt  n   argv 0        exit -1         else         char   const newstr   replace argv 1   argv 2   argv 3        if  newstr              printf   s n   newstr         free newstr             else             fprintf stderr  allocation error n          exit -2               return 0

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The optimizer should eliminate most of the local variables  The tmp pointer is there to make sure strcpy doesn t have to walk the string to find the null  tmp points to the end of result after each call   See Shlemiel the painter s algorithm for why strcpy can be annoying       You must free the result if result is non-NULL  char  str replace char  orig  char  rep  char  with        char  result     the return string     char  ins        the next insert point     char  tmp        varies     int len rep      length of rep  the string to remove      int len with     length of with  the string to replace rep with      int len front     distance between rep and end of last rep     int count        number of replacements         sanity checks and initialization     if   orig     rep          return NULL      len rep   strlen rep       if  len rep    0          return NULL     empty rep causes infinite loop during count     if   with          with           len with   strlen with           count the number of replacements needed     ins   orig      for  count   0  tmp   strstr ins  rep     count            ins   tmp   len rep             tmp   result   malloc strlen orig     len with - len rep    count   1        if   result          return NULL          first time through the loop  all the variable are set correctly        from here on            tmp points to the end of the result string           ins points to the next occurrence of rep in orig           orig points to the remainder of orig after  end of rep      while  count--            ins   strstr orig  rep           len front   ins - orig          tmp   strncpy tmp  orig  len front    len front          tmp   strcpy tmp  with    len with          orig    len front   len rep     move to next  end of rep            strcpy tmp  orig       return result

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Here goes mine  make them all char   which makes calling easier     char  strrpc char  str char  oldstr char  newstr       char bstr strlen str        memset bstr 0 sizeof bstr        int i      for i   0 i  lt  strlen str  i             if  strncmp str i oldstr strlen oldstr                 strcat bstr newstr               i    strlen oldstr  - 1           else                  strncat bstr str   i 1                            strcpy str bstr       return str

[c] What function is to replace a substring from a string in C?

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