[рони]

ufaclub,

let value="1x0.038x0.025";
let n = '12345';
let reg = /.*?(?=x)/;
value = value.replace(reg, n);
alert(value);

let value="1x0.038x0.025";
let n = '12345';
let ar = value.split("x");
ar[0] = n;
value = ar.join("x")
alert(value);