Question: How can I find all files from a folder and its subfolders recursively, containing a regexp in the filename, using helm?

For example: how to search for all files with the extension `.el` and containing the string `dired` in the directory `emacs-24.3` its subdirectories, using helm?

Answer: 
On systems that provide GNU `find` utility, you can use `helm-find` (by default bound to `C-x c /`) to get the desired behavior. Starting `helm-find` with a prefix argument `C-u C-x c /` prompts for the directory (`emacs24.3` in the example) first and then for the pattern (`dired el` in the example). Without the prefix argument, it uses the default directory of the current buffer.

EDIT: Note that `helm-find` cannot use wildcards or regexps unlike the actual `find` utility. It does however support a whitespace separator to match multiple pattern within a string. For instance `dired .el` matches only filenames that contain both the strings `dired` and `.el` in the path in any order.
This answer isn't exactly what you want (it does not use helm and does not use
regexp) but Emacs has a built-in command called `find-name-dired` which finds
recursively in a directory all files matching a given [shell wildcard](http://www.linfo.org/wildcard.html) and
opens a Dired buffer with all the matching files.

For your particular example you can do

`M-x` `find-name-dired` `RET` `/path/to/emacs-24.3` `RET` `*.el` `RET`