Bug in regex engine
#### Code Sample

```python
# This one fails
import pandas as pd
import numpy as np
import io
data = io.StringIO(r"""401.000000  383.1216 0.0000 0.0000 0.5140 0.4860   MASLENN 116616 NH3N2         1
401.000000  383.1216 0.0000 0.0000 0.5140 0.4860   MASLENN 116616 NH3N2         1""")
df = pd.read_csv(data, sep=r'\w+', engine='python', names=['T/K','p/MPa','x0','x1','y0','y1','ref','refnum','pair','index'])
print(np.log10(df['p/MPa']))
```
with error:

```
Traceback (most recent call last):
  File "D:\Code\bug.py", line 7, in <module>
    print(np.log10(df['p/MPa']))
AttributeError: 'str' object has no attribute 'log10'
```

```python
# This one succeeds
import pandas as pd
import numpy as np
import io
data = io.StringIO(r"""401.000000,383.1216,0.0000,0.0000,0.5140,0.4860,MASLENN,116616,NH3N2,1
401.000000,383.1216,0.0000,0.0000,0.5140,0.4860,MASLENN,116616,NH3N2,1""")
df = pd.read_csv(data, sep=',', names=['T/K','p/MPa','x0','x1','y0','y1','ref','refnum','pair','index'])
print(np.log10(df['p/MPa']))
```

#### Problem description

There is a very odd bug in pandas regarding the use of the python engine in concert with a regex.  Somehow this combination corrupts numpy <-> pandas interaction; note how python thinks that numpy is a string above.  I finally was able to put together a runnable example showing the problem as detailed above.  When I modify my example to use comma delimiters instead, there is no corruption of numpy

#### Output of pd.show_versions()

``` python
Python 3.6.0 |Continuum Analytics, Inc.| (default, Dec 23 2016, 11:57:41) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import pandas as pd
>>> pd.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Windows
OS-release: 7
machine: AMD64
processor: Intel64 Family 6 Model 58 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.4
pytest: None
pip: 19.0.3
setuptools: 27.2.0
Cython: 0.29.10
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.1.0
sphinx: 1.7.2
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 3.0.0
openpyxl: 2.4.8
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: 4.1.1
bs4: 4.7.1
html5lib: 0.999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```