[monolithed]

Можно так:

var replace = /абв/;
var subject = "абв";
alert(subject.replace(replace, "abv"))

А можно и так:

function str_replace (search, replace, subject ) {
    if(!(replace instanceof Array)){
        replace=new Array(replace);
        if(search instanceof Array){
            while(search.length>replace.length){
                replace[replace.length]=replace[0];
            }
        }
    }
    if(!(search instanceof Array))search=new Array(search);
    while(search.length>replace.length){
        replace[replace.length]='';
    }
    if(subject instanceof Array){
        for(k in subject){
            subject[k]=str_replace(search,replace,subject[k]);
        }
        return subject;
    }
    for(var k=0; k<search.length; k++){
        var i = subject.indexOf(search[k]);
        while(i>-1){
            subject = subject.replace(search[k], replace[k]);
            i = subject.indexOf(search[k],i);
        }
    }
    return subject;
}

alert(str_replace('абв ', 'абв', 'abv'));