Bug #480: problem with LIKE pattern matches involving % and \_

Tom Cross (decius(at)whack(dot)org) reports a bug with a severity of 2
The lower the number the more severe it is.

Short Description
problem with LIKE pattern matches involving % and \_

Long Description
The following pattern matches do not work:

LIKE '%\_%'
LIKE '%\_'
LIKE '\_%'

They seem to match everything even if it does not contain an underscore. There seems to be no problem if another character
is involved somehow, such as LIKE '\_b%'

I'm running 7.1.3...

Sample Code
somedatabase=# select * from tom;
stuff
-----------
blah
blah_
_blah
blah_blah
(4 rows)

somedatabase=# select * from tom where stuff LIKE '%\_%';
stuff
-----------
blah
blah_
_blah
blah_blah
(4 rows)

somedatabase=# select * from tom where stuff LIKE '\_%';
stuff
-----------
blah
blah_
_blah
blah_blah
(4 rows)

somedatabase=# select * from tom where stuff LIKE '%h\_%';
stuff
-----------
blah_
blah_blah
(2 rows)

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