series.replace bug in .24 - regex not replacing patterns in middle of string (only beginning)
#### Code Sample, a copy-pastable example if possible

###sample code:
import pandas as pd
import re   

s = pd.Series(['hello there', 'well hello then'])
print(s)

'''OUTPUT
0        hello there
1    well hello then
dtype: object
'''

s.replace({'hello' : 'goodbye'}, regex=True, inplace=True)

print(s)

'''OUTPUT
0      goodbye there
1    well hello then  #<-not replaced
dtype: object
'''

###expected output using re.sub as per series.replace documentation
x = re.sub('hello','goodbye', 'hello there')
print(x) 
#out: goodbye there

x = re.sub('hello','goodbye', 'well hello then')
print(x)


#### Problem description
.24 bug where series.replace is no longer working the same as in .23 pandas. In past versions, pandas would replace substrings that appeared in the middle/end of strings, but now it only seems to work if the pattern is at the very beginning.

I saw the docs mention that passing a dict and using regex follows re.sub, but when I tried that, I got the expected result (not the one .24 pandas yielded). 

If the issue has not been resolved there, go ahead and file it in the issue tracker.

#### Expected Output
see above - should replace sub-strings in middle (not just beginning)

#### Output of ``pd.show_versions()``
<details>
[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.7.0.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.24.1
pytest: 3.8.0
pip: 10.0.1
setuptools: 40.2.0
Cython: 0.28.5
numpy: 1.16.1
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.5.0
sphinx: 1.7.9
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: None
numexpr: None
feather: None
matplotlib: 2.2.3
openpyxl: 2.5.6
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.1.0
lxml.etree: None
bs4: 4.6.3
html5lib: 1.0.1
sqlalchemy: 1.2.11
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
None
</details>
