The most basic rewriting system to play with

rewriter.js

//
//  Copyright 2018-present by Pouya Kary. Do whatever you like with
//  it and don't sue me :D
//
//  This is the most simple rewriting system that one can make.
//  it might look silly but you can experiment all sorts of cool
//  systems in it. Just check out the sample rules and the result
//  you're going to get. Believe it or not it was only an accident
//  to get that result.
//
//  BTW, Do you need a source of inspirations?
//      * https://www.youtube.com/watch?v=JbseluMTfZQ
//      * https://www.wolframscience.com/nks/p102--symbolic-systems/
//
//  Also, the runtime has a maximum iteration variable to keep it
//  from falling to infinity replacement rule. You can change it to
//  whatever you want as long as you're happy with it.
//

//
// ─── RULES ──────────────────────────────────────────────────────────────────────
//

    function get_rules ( code ) {
        const rules =
            code.trim( )
                .split("\n")
                .map( rule => rule.split( "->" ).map( x => x.trim( ) ) )

        const resulting_rules = [ ]
        for ( const rule of rules )
            resulting_rules.push(
                text => text.replace( new RegExp( rule[ 0 ], "g" ), rule[ 1 ] )
            )

        return resulting_rules
    }

//
// ─── EVALUATE ───────────────────────────────────────────────────────────────────
//

    function evaluate ( rules_code, code ) {
        const rules = get_rules( rules_code )
        const max_iteration = 1000
        let iteration = 1

        while ( iteration <= max_iteration ) {
            let new_code = null
            console.log( "--->", code )

            for ( const rule of rules )
                new_code = rule( new_code? new_code : code )

            if ( new_code == code )
                return new_code
            else
                code = new_code
        }
    }

//
// ─── MAIN ───────────────────────────────────────────────────────────────────────
//

    // Edit these rules and remember that the parser is a joke so
    // don't have errors!

    const rules = (`
        -7 -> 62
        17 -> 72
        27 -> 82
        -8 -> 61
        18 -> 71
        28 -> 81
        6- -> 23
        61 -> 24
        62 -> 25
        4- -> -6
        41 -> -7
        42 -> -8
        5- -> 13
        51 -> 14
        52 -> 15
        -3 -> 61
        13 -> 71
        23 -> 81
    `)

                     // this is a text
                     // that all the rules
                     // are applied at
    evaluate( rules, "-----------------6---------------" )

    //
    //  P.S: What you're about to witness is the world's smallest
    //         working and proven Turing Machine.
    //

// ────────────────────────────────────────────────────────────────────────────────