The official VisuStella notetag help thread

/*:
* @plugindesc Regen Flattener
* @author mtat
* @help
* Use notetags <flat hrg x> and <flat mrg x> in any state.
* Replace "x" with a number. Use negative numbers for damage.
* ex: <flat hrg -10> will deal 10 damage every turn.
*/

Game_Battler.prototype.regenerateHp = function () {
    // set initial value
    let value = this.mhp * this.hrg;
    // add values when notetag is detected
    for (i = 0; i < this.states().length; i++) {
        let note = $dataStates[this.states()[i].id].note;
        let notetag = /(?:<flat\shrg\s(-?\d+)>)/i.exec(note);
        if (notetag) {
            console.log(notetag[1]);
            value += parseInt(notetag[1]);
        }
    }
    // max slip damage (depends on the options in "System 1" in the database)
    value = Math.max(value, -this.maxSlipDamage());
    // apply to actor
    if (value !== 0) {
        this.gainHp(Math.round(value));
    }
};
Game_Battler.prototype.regenerateMp = function () {
    // set initial value
    let value = this.mmp * this.mrg;
    // add values when notetag is detected
    for (i = 0; i < this.states().length; i++) {
        let note = $dataStates[this.states()[i].id].note;
        let notetag = /(?:<flat\smrg\s(-?\d+)>)/i.exec(note);
        if (notetag) {
            value += parseInt(notetag[1]);
        }
    }
    // apply to actor
    if (value !== 0) {
        this.gainMp(Math.round(value));
    }
};