I have many files with names in the form 

```
username_pattern_suffix.ext  

```

where "pattern" is fixed, but there are very many usernames, each with several files (i.e. values of 'suffix'). I'd like concatenate each user's files into a single file, e.g.

```
username.ext

```

to end up with one file per username. Each username contains two underscores`_` and ends with a number, but is variable length.