Замена по регулярному выражению использую pcre

Думаю, да. Судя по нижеследующему.

REPLACING PARTS OF STRINGS

       You can replace the first match of "pattern" in "str"  with  "rewrite".
       Within  "rewrite",  backslash-escaped  digits (\1 to \9) can be used to
       insert text matching corresponding parenthesized group  from  the  pat-
       tern. \0 in "rewrite" refers to the entire matching text. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").Replace("d", &s);

       will  leave  "s" containing "yada dabba doo". The result is true if the
       pattern matches and a replacement occurs, false otherwise.

       GlobalReplace is like Replace except that it replaces  all  occurrences
       of  the  pattern  in  the string with the rewrite. Replacements are not
       subject to re-matching. For example:

         string s = "yabba dabba doo";
         pcrecpp::RE("b+").GlobalReplace("d", &s);

       will leave "s" containing "yada dada doo". It  returns  the  number  of
       replacements made.

       Extract  is like Replace, except that if the pattern matches, "rewrite"
       is copied into "out" (an additional argument) with substitutions.   The
       non-matching  portions  of "text" are ignored. Returns true iff a match
       occurred and the extraction happened successfully;  if no match occurs,
       the string is left unaffected.

Забыл сказать: язык - чистый Си

Я не припоминаю в pcrelib функций замены, так что используй полученные смещения из массива возвращенного pcre_exec-ом.