Update arXiv regex

Simplifies the arXiv matching regex to make it more readable and to allow extracting the id with version number, without version number or just the version number. The multiple capture groups are overkill, but the regex might be helpful for future changes of the arXiv translator.

utilities.js

@@ -430,13 +430,12 @@ var Utilities = {
 
 		// Next try arXiv
 		if (!identifiers.length) {
-			// arXiv identifiers are extracted without version number
-			// i.e. 0706.0044v1 is extracted as 0706.0044,
-			// because arXiv OAI API doesn't allow to access individual versions
-			let arXiv_RE = /((?:[^A-Za-z]|^)([\-A-Za-z\.]+\/\d{7})(?:(v[0-9]+)|)(?!\d))|((?:\D|^)(\d{4}\.\d{4,5})(?:(v[0-9]+)|)(?!\d))/g;
+			// arXiv identifiers are extracted with and without version number
+			// i.e. 0706.0044v1 is extracted as [0706.0044v1, 0706.0044, 1]
+			let arXiv_RE = /\b(([-A-Za-z.]+\/\d{7}|\d{4}\.\d{4,5})(?:v(\d+))?)(?!\d)/g; // 1: full ID, 2: ID without version, 3: version #
 			let m;
 			while ((m = arXiv_RE.exec(text))) {
-				let arXiv = m[2] || m[5];
+				let arXiv = m[1];
 				if (arXiv && !foundIDs.has(arXiv)) {
 					identifiers.push({arXiv: arXiv});
 					foundIDs.add(arXiv);