google chrome - javascript indexOf with millions of matches -

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i'm trying extract few lines representing xml elements file. user provides file using simple <input type="file"> tag, , file read text filereader, , given parameter function:

var relevantdelimiters = [{"begin":"<header>","end":"</header>"} ,{"begin":" <someelement>","end":"</someelement>"}];  function dealwithstring(invalidxml) {   var validxml = "";   (var i=0; i<relevantdelimiters.length; i++) {     delimiter = relevantdelimiters[i];     while (invalidxml.indexof(delimiter.begin) != -1) {       //while there relevant elements of kind left:        startpos = invalidxml.indexof(delimiter.begin);       endpos = invalidxml.indexof(delimiter.end);        //append end result:       validxml+=invalidxml.substring(startpos,endpos+delimiter.end.length)+"\n";       //take item out of input process next item       invalidxml = invalidxml.replace(invalidxml.substring(startpos,endpos+delimiter.end.length),"");     }   }   //return fixed data   return validxml; }

this approach seems work fine small amount of matches in input text file, given file of 1.5mb, script stuck (running google chrome, making it's tab non-responsive). file contains million "relevant elements", meaning matches relevantdelimiters.

how can optimize this?

instead of repeatedly "taking item out of input" calling replace on it, should use second argument indexof: fromindex. way, it'll search next occurence after given index, , can loop through large input without needing touch it.

function dealwithstring(invalidxml) {   var validxml = "";   (var i=0; i<relevantdelimiters.length; i++) {     var delimiter = relevantdelimiters[i],         pos = 0,         startpos;     while ((startpos = invalidxml.indexof(delimiter.begin, pos)) != -1) {       //while there relevant elements of kind left:       var endpos = invalidxml.indexof(delimiter.end, startpos);       // assert(endpos != -1) - otherwise go horribly wrong       pos = endpos+delimiter.end.length;       //append end result:       validxml += invalidxml.slice(startpos, pos) + "\n";     }   }   return validxml; }

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