Revision d9f29f40-27e0-4de9-9eb1-4a03e0938a9d - Code Golf Stack Exchange

# Python (<strike>337</strike> 336 chars)

 import re
 i=input().split()
 a,d,o,e=[],{},"",len(i)-1
 for b in i:a.append(dict(re.findall(r"([A-Z][a-z]*)(\d*)",b)))
 a=[{b:int(c[b])if c[b]else 1for b in c}for c in a]
 for b in a:
 for c in b:d[c]=c in d and d[c]+b[c]or b[c] 
 if"H"not in b:d["-"]=0
 d["H"]-=2*e;d["O"]-=e
 for b in d:o+=b+(str(d[b])if d[b]-1else"")if d[b]else""
 print(o)
Here is an example of how to use this program:

 sh3.14$ python dehydrate.py <-Run the program
 C6H12O6 HgHO4 <-Input
 O9H11C6Hg <-Output
[Try it!](https://repl.it/Bx5D/0)