DataFrame regex with dictionary will not work correctly.
#### Code Sample

```python
df = pd.DataFrame(
    {'A': [0, 1, 2],
     'B': ['ba\nt', 'foo', 'bait'],
     'C': ['abc', 'ba\nr', 'xyz']}
)
df.replace(regex={'\n': '', r'^fo.$': 'xyz'})  # Neither do r'\n' or '\\n' work.
```
#### Problem description

Does not replace the newline character. Also checked with other escaped characters like spaces, tabs. However, the other regex expression works:

```python
	A	B	C
0	0	ba\nt	abc
1	1	xyz	ba\nr
2	2	bait	xyz
```

#### Expected Output

```python
	A	B	C
0	0	bat	abc
1	1	xyz	bar
2	2	bait	xyz
```

#### Additional information

Working:
```python
df.replace('\n', '', regex=True).replace('foo', 'xyz')
```

[Related SO question](https://stackoverflow.com/q/55190441/3765319).

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 3.7.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.1
pytest: None
pip: 19.0.3
setuptools: 40.8.0
Cython: 0.29.5
numpy: 1.15.4
scipy: 1.1.0
pyarrow: None
xarray: 0.11.3
IPython: 7.3.0
sphinx: None
patsy: 0.5.1
dateutil: 2.8.0
pytz: 2018.9
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.9
feather: None
matplotlib: 3.0.3
openpyxl: 2.6.0
xlrd: 1.2.0
xlwt: 1.3.0
xlsxwriter: 1.1.5
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: 1.2.18
pymysql: None
psycopg2: 2.7.6.1 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None


</details>
