Very tiny APP

Right now I'm learning languages and I have an idea for a small application. I want software that "spoils" the .srt file of subtitles of a movie or series, for me it is very easy to see a film in English with English subtitles, and difficult to see a movie without subtitles. The software turns this: 1 00:02:12,087 --> 00:02:13,964 - Jump! - What are you doing? 2 00:02:24,727 --> 00:02:26,718 - Oh, my God! - What are you doing? 3 00:02:28,527 --> 00:02:29,721 Help me! In this: 1 00:02:12,087 --> 00:02:13,964 - Jump! - ____ are you doing? 2 00:02:24,727 --> 00:02:26,718 - Oh, __ God! - What ___ you ______ 3 00:02:28,527 --> 00:02:29,721 ____ me! The application scrolls through each word of the text respecting everything that has numbers and the arrow "-->", every time it goes through a word it throws a random cast to decide if it modifies is word or not, if it modifies it changes all its characters by " _ ". There is another mode that does not delete the whole word but if it modifies several letters of the word, becoming "doing?" In "d__ng_". For this the system launches another random cast to decide if it turns each character for “_”. The necessary parameters will be: Rate of modifying word: (0-1) Probability with which the software modifies each word, with 0 no words will change with 1 all will do. Ratio of modifying only letters: (0-1) Each time a word is pointed to be modified, it will launch a random cast, to decide if it modifies all its characters or only a part of them. With 0 all the words to modify will be modified completely, with 1 all the words to modify will be modified by the method letter by letter. Rate of modifying characters: (0-1) Probability with which a caracter is modified in letter-only mode, if a word is pointed to modify and is decide to modify by letter by letter mode, the code will cast a random cast for each letter to be turned in a “_”. Turning "doing?" in "d__ng_". The same application can be used to create "fill the gap" exercises with common text in .txt. I want a last option to erase all the “intro” in the text. I want the APP in raw and simple code, I prefer javascritp, with some comments, the APP will take the .srt and .txt files near it and it will create new files with the modification. I have found libraries to parse .srt [login to view URL] [login to view URL]