extracting numbers from a file, excluding words

[dawenliu at gmail.com]

Hi, I have a file with this content:

zzzz zzzzz zzz zzzzz
...
xxxxxxx xxxxxxxxxx xxxxx 34.215
zzzzzzz zz zzzz
...

"x" and "z" are letters.
The lines with "z" are trash, and only the lines with "x" are
important.  I want to extract the number (34.215 in this case) behind
the letters x, and store it in a file.

The sentence "xxxxxxx xxxxxxxxxx xxxxx " is FIXED and KNOWN. The "z"
sentences are can vary.  There are also unknown number of "z" lines.

Any suggestions will be appreciated.