[YouTube] Don't use a specific letter for the decryption function name pattern

Use the same possible characters for variables everywhere, in order to avoid
potential future throttling parameter decryption function name parsing issues
related to the usage of other letter(s) than b.

Author: AudricV

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeThrottlingDecrypter.java

@@ -38,7 +38,9 @@ public final class YoutubeThrottlingDecrypter {
 
     private static final Pattern N_PARAM_PATTERN = Pattern.compile("[&?]n=([^&]+)");
     private static final Pattern DECRYPT_FUNCTION_NAME_PATTERN = Pattern.compile(
-            "\\.get\\(\"n\"\\)\\)&&\\(b=([a-zA-Z0-9$]+)(?:\\[(\\d+)])?\\([a-zA-Z0-9]\\)");
+            // CHECKSTYLE:OFF
+            "\\.get\\(\"n\"\\)\\)&&\\([a-zA-Z0-9$_]=([a-zA-Z0-9$_]+)(?:\\[(\\d+)])?\\([a-zA-Z0-9$_]\\)");
+            // CHECKSTYLE:ON
 
     // Escape the curly end brace to allow compatibility with Android's regex engine
     // See https://stackoverflow.com/q/45074813