Format txt file?

Hello everyone, I have a txt file that looks like this: 1231238: UNIMPORTANT NUMBERS: UNIMPORTANT LETTERS: ETC. 1562326: UNIMPORTANT NUMBERS: UNIMPORTANT LETTERS: ETC. 123987: UNIMPORTANT NUMBERS: UNIMPORTANT LETTERS: ETC. … thousands more such lines However, I only need the first number in each row. So, the first "column" (if there were columns), so to speak….