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~~# [Python](https://python.org) (344 bytes)~~
**534 bytes**:
This is _not_ golfed in any way. But it's the current winner!
``` python
def validate_roman_numeral(candidate):
invalid_strings = ["CCCC", "CCD", "CCM", "CDC", "CMC", "CMD", "CMM", "DCD", "DCM", "DD", "DM", "IC", "ID", "IIII", "IIV", "IIX", "IL", "IM", "IVI", "IXC", "IXI", "IXL", "IXV", "IXX", "LC", "LD", "LL", "LM", "LXC", "LXL", "MMMM", "VC", "VD", "VIV", "VIX", "VL", "VM", "VV", "VX", "XCC", "XCD", "XCL", "XCM", "XCX", "XD", "XLX", "XM", "XXC", "XXL", "XXXX"]
for invalid_string in invalid_strings:
if invalid_string in candidate:
return invalid_string
return "VALID"
```
**379 bytes**:
Here's a minimal effort golf of the above:
``` python
def v(c):
i=['DD','DM','IC','ID','IL','IM','LC','LD','LL','LM','VC','VD','VL','VM','VV','VX','XD','XM','CCD','CCM','CDC','CMC','CMD','CMM','DCD','DCM','IIV','IIX','IVI','IXC','IXI','IXL','IXV','IXX','LXC','LXL','VIV','VIX','XCC','XCD','XCL','XCM','XCX','XLX','XXC','XXL','CCCC','IIII','MMMM','XXXX']
for s in i:
if s in c:
return s
return "VALID"
```
**344 bytes**:
A little more effort:
``` python
def v(c):
o="VALID"
for s in 'DD','DM','IC','ID','IL','IM','LC','LD','LL','LM','VC','VD','VL','VM','VV','VX','XD','XM','CCD','CCM','CDC','CMC','CMD','CMM','DCD','DCM','IIV','IIX','IVI','IXC','IXI','IXL','IXV','IXX','LXC','LXL','VIV','VIX','XCC','XCD','XCL','XCM','XCX','XLX','XXC','XXL','C'*4,'I'*4,'M'*4,'X'*4:
if s in c:
o=s
return o
```

# [Python](https://python.org), 344 bytes
**534 bytes**:
This is _not_ golfed in any way. But it's the current winner!
``` python
def validate_roman_numeral(candidate):
invalid_strings = ["CCCC", "CCD", "CCM", "CDC", "CMC", "CMD", "CMM", "DCD", "DCM", "DD", "DM", "IC", "ID", "IIII", "IIV", "IIX", "IL", "IM", "IVI", "IXC", "IXI", "IXL", "IXV", "IXX", "LC", "LD", "LL", "LM", "LXC", "LXL", "MMMM", "VC", "VD", "VIV", "VIX", "VL", "VM", "VV", "VX", "XCC", "XCD", "XCL", "XCM", "XCX", "XD", "XLX", "XM", "XXC", "XXL", "XXXX"]
for invalid_string in invalid_strings:
if invalid_string in candidate:
return invalid_string
return "VALID"
```
**379 bytes**:
Here's a minimal effort golf of the above:
``` python
def v(c):
i=['DD','DM','IC','ID','IL','IM','LC','LD','LL','LM','VC','VD','VL','VM','VV','VX','XD','XM','CCD','CCM','CDC','CMC','CMD','CMM','DCD','DCM','IIV','IIX','IVI','IXC','IXI','IXL','IXV','IXX','LXC','LXL','VIV','VIX','XCC','XCD','XCL','XCM','XCX','XLX','XXC','XXL','CCCC','IIII','MMMM','XXXX']
for s in i:
if s in c:
return s
return "VALID"
```
**344 bytes**:
A little more effort:
``` python
def v(c):
o="VALID"
for s in 'DD','DM','IC','ID','IL','IM','LC','LD','LL','LM','VC','VD','VL','VM','VV','VX','XD','XM','CCD','CCM','CDC','CMC','CMD','CMM','DCD','DCM','IIV','IIX','IVI','IXC','IXI','IXL','IXV','IXX','LXC','LXL','VIV','VIX','XCC','XCD','XCL','XCM','XCX','XLX','XXC','XXL','C'*4,'I'*4,'M'*4,'X'*4:
if s in c:
o=s
return o
```