ENH: column label filtering via regexes to work for numeric names
Simple fix to allow regex filtering to work for numeric column labels, e.g. df.filter(regex="[12][34]")

closes #10506 

can you add some tests?
 For search(x) -> search(str(x))?
 Any advice on what to add or where? I don't see any existing tests for this function...
 look in `pandas/tests/test_frame` for `test_filter`
 Thanks Jeff! Added the test. Let me know what you think...
 add a not in whatsnew/0.17.0. Put in Other Enhancements section

What would this do in 0.16.2 (if you passed the regex), not fitler anything? or raise?
 Done! In 0.16.2 `re.search` will raise if a column name is numeric...
 when you are all done, pls rebase/squash see contributing docs [here](http://pandas.pydata.org/pandas-docs/stable/contributing.html)
 I'm having trouble with squashing the commits. I don't have a ton of experience with git, so I'm not sure what to do next. Below is the message. Seems to have to do with a merge conflict in test_frame? Any advice?

```
error: could not apply ac90352... Add test for regex filter on numeric column names

When you have resolved this problem, run "git rebase --continue".
If you prefer to skip this patch, run "git rebase --skip" instead.
To check out the original branch and stop rebasing, run "git rebase --abort".
```
 contributing docs are here: http://pandas.pydata.org/pandas-docs/stable/contributing.html

you have a conflict and need to fix it
 Hmm, when I rebase it detects conflicts, then I resolve them using `git mergetool`, and commit. Doesn't seem to change anything. When I run `git merge master` I get that everything is up-to-date. I'm probably missing something simple?
 FYI, you don't normally need to add an issue if you just create a PR (like you did), but no biggie.
 I rebase you: https://travis-ci.org/jreback/pandas/builds/69631109

FYI don't use merge master. This is not pandas standard practice. This makes rebasing much more difficult.
 merged via https://github.com/pydata/pandas/commit/bfe5a7f0c1d7250be68f1d2bceca2138d561014d

thanks!
