diff --git a/base/mp_replace.sas b/base/mp_replace.sas
index 92825a2..d20e68e 100644
--- a/base/mp_replace.sas
+++ b/base/mp_replace.sas
@@ -11,29 +11,31 @@
 
   Usage:
 
-      %let file=%sysfunc(pathname(work))/file.txt;
+      %let file="%sysfunc(pathname(work))/file.txt";
       %let str=replace/me;
       %let rep=with/this;
       data _null_;
-        file "&file";
+        file &file;
         put 'blahblah';
         put "blahblah&str.blah";
         put 'blahblahblah';
       run;
-      %mp_replace(file=&file, patternvar=str, replacevar=rep)
+      %mp_replace(&file, findvar=str, replacevar=rep)
       data _null_;
-        infile "&file";
+        infile &file;
         input;
         list;
       run;
 
-  @param file= (0) The file to perform the substitution on
-  @param patternvar= Macro variable NAME containing the string to search for
+  @param infile The QUOTED path to the file on which to perform the substitution
+  @param findvar= Macro variable NAME containing the string to search for
   @param replacevar= Macro variable NAME containing the replacement string
-  @param outfile= (mp_rplce) The output fileref to contain the adjusted file.
+  @param outfile= (0) Optional QUOTED path to an the adjusted output file (to
+    avoid overwriting the first file).
 
   <h4> SAS Macros </h4>
-  @li ml_gsubfile.sas
+  @li mf_getuniquefileref.sas
+  @li mf_getuniquename.sas
 
   <h4> Related Macros </h4>
   @li mp_gsubfile.test.sas
@@ -42,12 +44,111 @@
   @author Allan Bowe
 **/
 
-%macro mp_replace(file=0,
-  patternvar=,
+%macro mp_replace(infile,
+  findvar=,
   replacevar=,
-  outref=mp_rplce
+  outfile=0
 )/*/STORE SOURCE*/;
 
-  %ml_gsubfile()
+%local inref dttm ds1;
+%let inref=%mf_getuniquefileref();
+%let outref=%mf_getuniquefileref();
+%if &outfile=0 %then %let outfile=&infile;
+%let ds1=%mf_getuniquename(prefix=allchars);
+%let ds2=%mf_getuniquename(prefix=startmark);
+
+/* START */
+%let dttm=%sysfunc(datetime());
+
+filename &inref &infile lrecl=1 recfm=n;
+
+data &ds1;
+  infile &inref;
+  input x $ 1. @@;
+  format x hex2.;
+run;
+
+data &ds2;
+
+  /* set find string to length in bytes to cover trailing spaces */
+  length string $ %length(%superq(&findvar));
+  string =symget("&findvar");
+  drop string;
+
+  z =char(string,1);
+  l =lengthm(string); /* <- for trailing bytes */
+
+  do _N_ =1 to nobs;
+    set &ds1 nobs=nobs point=_N_;
+    if x =z then do;
+      pos =1;
+      s =0;
+      do point =_N_ to min(_N_ + l -1,nobs);
+        set &ds1 point=point;
+        if x =char(string, pos) then s + 1;
+        else goto _leave_;
+        pos+1;
+      end;
+      _leave_:
+      if s =l then do;
+        START =_N_;
+        _N_ =_N_+ s - 1;
+        STOP =_N_;
+        output;
+      end;
+      else _N_ =_N_+ s;
+    end;
+  end;
+  stop;
+  keep START STOP;
+run;
+
+data &ds1;
+  declare hash HS(dataset:"&ds2(keep=start)");
+  HS.defineKey("start");
+  HS.defineDone();
+
+  declare hash HE(dataset:"&ds2(keep=stop)");
+  HE.defineKey("stop");
+  HE.defineDone();
+
+  do until(eof);
+    set &ds1 end=eof curobs =n;
+
+    start=0;
+    stop=0;
+    if 0 =HS.check(key:n) then start =1;
+    if 0 =HE.check(key:n) then stop  =1;
+
+    length strt $ 1;
+    strt =put(start,best. -L);
+
+    retain out 1;
+    if out   then output;
+    if start then out=0;
+    if stop  then out=1;
+  end;
+
+  stop;
+
+  keep x strt;
+run;
+
+filename &outref &outfile recfm=n;
+
+data _null_;
+  length replace $ %length(%superq(&replacevar));
+  replace=symget("&replacevar");
+  file &outref;
+  do until(eof);
+    set &ds1 end=eof;
+    if strt ="1" then put replace char.;
+    else put x char1.;
+  end;
+  stop;
+run;
+
+/* END */
+%put &sysmacroname took %sysevalf(%sysfunc(datetime())-&dttm) seconds to run;
 
 %mend mp_replace;