MtgDeckCard.jsx

class MtgCard {
/*	raw;
	quantity;
	name = '';
	set = '';
	setNumber = 0;
*/
	constructor( input = '' ) {
		this.raw = '';

		this.quantity = 0;
		this.name = '';
		this.set = '';
		this.setNumber = '';

		this.parseImport( input );
	}

	parseImport( input ) {
		if ( ! input ) {
			return;
		}

		this.raw = input;

		const matches = input.match( /^(?<quantity>\d+) (?<name>.+) \((?<set>[\da-zA-Z]{3})\) (?<setNumber>\d+)$/ );

		if ( matches ) {
			this.quantity  = matches.groups.quantity;
			this.name      = matches.groups.name;
			this.set       = matches.groups.set;
			this.setNumber = matches.groups.setNumber;
		}
	}

	get arena() {
		return this.quantity + ' ' + this.name + ' (' + this.set + ') ' + this.setNumber;
	}
}

class MtgDeck {
/*	Companion;
	Commander;
	Deck = [];
	Sideboard = [];
*/
	constructor( input = false ) {
		this.Deck = [];
		this.Sideboard = [];

		if ( input ) {
			this.parseImport( input );
		}
	}

	parseImport( input ) {
		if ( ! Array.isArray( input ) ) {
			input = input.split('\n');
		}

		let type = 'Deck';

		input.forEach( function( line ) {
			if ( line ) {
				// If it starts with a digit, it's a card.  Else, it's a type.
				if ( line.match( /^\d/ ) ) {
					// Decks and Sideboards can be multiple cards, so add it to the array.
					if ( 'Deck' === type || 'Sideboard' === type ) {
						this[ type ].push( new MtgCard( line ) );
					} else {
						this[ type ] = new MtgCard( line );
					}
				} else {
					type = line;
				}
			}
		}, this );
	}

	get arena() {
		let deckExport = '';

		if ( this.Commander ) {
			deckExport += 'Commander\n' + this.Commander.arena + '\n\n';
		}

		if ( this.Companion ) {
			deckExport += 'Companion\n' + this.Companion.arena + '\n\n';
		}

		if ( this.Sideboard.length ) {
			deckExport += 'Deck\n';
			this.Deck.forEach( function( card ) {
				deckExport += card.arena + '\n';
			} )
			deckExport += '\n';
		}

		if ( this.Sideboard.length ) {
			deckExport += 'Sideboard\n';
			this.Sideboard.forEach( function( card ) {
				deckExport += card.arena + '\n';
			} )
			deckExport += '\n';
		}

		return deckExport;
	}
}

Usage.jsx

const deckString = `
Commander
1 Daxos, Blessed by the Sun (THB) 9

Companion
1 Lurrus of the Dream Den (IKO) 226

Deck
1 Stonecoil Serpent (ELD) 235
1 Healer's Hawk (GRN) 14
22 Plains (IKO) 262
1 Ajani's Pridemate (WAR) 4
1 Soulmender (M20) 37
1 Hunted Witness (GRN) 15
1 Charmed Stray (WAR) 8
1 Beloved Princess (ELD) 7
1 Alseid of Life's Bounty (THB) 1
1 Charming Prince (ELD) 8
1 Daybreak Chaplain (M20) 12
1 Gingerbrute (ELD) 219
1 Impassioned Orator (RNA) 12
1 Hushbringer (ELD) 18
1 Grateful Apparition (WAR) 17
1 Drannith Healer (IKO) 10
1 Haazda Marshal (GRN) 13
1 Sworn Companions (GRN) 27
1 Shadowspear (THB) 236
1 The Birth of Meletis (THB) 5
1 Sentinel's Mark (RNA) 20
1 Battlefield Promotion (WAR) 5
1 Dawn of Hope (GRN) 8
1 Desperate Lunge (WAR) 266
1 Moment of Heroism (M20) 30
1 Rally for the Throne (ELD) 25
1 Triumphant Surge (THB) 41
1 Take Heart (GRN) 28
1 Light of Hope (IKO) 20
1 Golden Egg (ELD) 220
1 Shatter the Sky (THB) 37
1 Citywide Bust (GRN) 4
1 Unbreakable Formation (RNA) 29
1 Ravnica at War (WAR) 28
1 Inspired Charge (M20) 24
1 Faerie Guidemother (ELD) 11
1 Solid Footing (IKO) 31
1 Sentinel's Eyes (THB) 36

Sideboard
1 Lurrus of the Dream Den (IKO) 226
`.trim();

const deck = new MtgDeck( deckString );

console.log( deck.arena );