Breaking after one of several string replaces occur
This seems to be a fine case for a loop over the expressions:

function format_input(input) {
    // decouple DOM things from pure functionality!
    input.value = format_number(input.value);
}
function format_number(str) {
    var replacements = [
        [/regex1/, 'replace1'],
        [/regex2/, 'replace2'],
        [/regex3/, 'replace3'],
        [/regex4/, 'replace4'],
        [/regex5/, 'replace5'],
        [/regex6/, 'replace6'],
        [/regex7/, 'replace7'],
        [/regex8/, 'replace8'],
        [/regex9/, 'replace9'],
        [/regex10/, 'replace10'],
        [/regex11/, 'replace11'],
        [/regex12/, 'replace12'],
        [/regex13/, 'replace13'],
        [/regex14/, 'replace14'],
        [/regex15/, 'replace15'],
        [/regex16/, 'replace16'],
        [/regex17/, 'replace17']
    ];
    for (var i=0; i&lt;replacements.length; i++) {
        var copy = str.replace(replacements[i][0], replacements[i][1]);
        if (str != copy)
            return copy;
    }
    return str;
}