php html输入文本字段

Question

The user sets through the field an inequality, i.e. 50<A, X>30, 50<C<100. This is stored through javascript function on button click.

My issue is, the field doesn't work in a specific case when there's a "less than" symbol followed by a letter. So, it will store fine e.g. X>30, A<50, but NOT 50<A or 50<C<100. In both of the latter cases it just shows the first number, meaning it's having trouble with the pattern "less than followed by letter", as this is simply omitted. How do I make it work?

Here's my code (php):

echo "<input type=\"text\" id=\"equation\">";
echo "<button onclick=\"addEq()\">Add</button>";

And javascript part (basically appends all equations entered, separated by ":")

var addEq = function() {
    eq = document.getElementById("equation").value;
    if (document.getElementById("equation").innerHTML == "") {
        document.getElementById("equation").innerHTML = eq;}
    else {
        document.getElementById("equation").innerHTML += ":" + eq;}}

Answer 1

Replace the < in your inequalities with < and > with >.
With < and a letter, the browser is trying to render it as html because it thinks it's a tag (like <a>).

var addEq = function() {
    eq = document.getElementById("equation").value.replace(/</g, '&lt;');
    if (document.getElementById("result").innerHTML == "") {
        document.getElementById("result").innerHTML = eq;
    } else {
        document.getElementById("result").innerHTML += ":" + eq;
    }
}
function getResult() {
    var res = document.getElementById("result").innerHTML.replace(/&lt;/g, '<');
    console.log(res);
    return res;
}

<input type="text" id="equation" value="50<C<100">
<button onclick="addEq()">Add</button>
<button onclick="getResult()">Result</button>
<span id="result"></span>

</div>