Ubuntu: how to install flex to make postgres?

I'm trying to work on postgresql local开发者_开发知识库ly with ubuntu 10.04. I get this error after I configure with

./configure --enable-cassert --enable-debug --prefix=$HOME/pgsql

when I make && make install I get this error:

make -C parser all
make[3]: Entering directory `/home/james/school/db/pg-stuff/hw1/postgresql-8.4.2/src/backend/parser'

ERROR: `flex' is missing on your system. It is needed to create the
file `scan.c'. You can either get flex from a GNU mirror site
or download an official distribution of PostgreSQL, which contains
pre-packaged flex output.

make[3]: *** [scan.c] Error 1
make[3]: Leaving directory `/home/james/school/db/pg-stuff/hw1/postgresql-8.4.2/src/backend/parser'
make[2]: *** [parser-recursive] Error 2
make[2]: Leaving directory `/home/james/school/db/pg-stuff/hw1/postgresql-8.4.2/src/backend'
make[1]: *** [all] Error 2
make[1]: Leaving directory `/home/james/school/db/pg-stuff/hw1/postgresql-8.4.2/src'
make: *** [all] Error 2

I tried to

apt-get install flex but I get the same error. How can I install flex so that this will work?

You need to run ./configure again so that the make files are "aware" of flex.

You can install postgresql directly using:

sudo apt-get install postgresql-8.4

I don't understand how you could get the same error if you ran:

sudo apt-get install flex

mypc:~/> sudo apt-get install flex

Reading package lists... Done

Building dependency tree

Reading state information... Done

Suggested packages:

bison

The following NEW packages will be installed:

flex

0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.

Need to get 261kB of archives.

After this operation, 1,032kB of additional disk space will be used.

Get:1 http://us.archive.ubuntu.com/ubuntu/ lucid/main flex 2.5.35-9 [261kB]

Fetched 261kB in 1s (247kB/s)

Selecting previously deselected package flex.

(Reading database ... 208958 files and directories currently installed.)

Unpacking flex (from .../flex_2.5.35-9_amd64.deb) ...

Processing triggers for install-info ...

Processing triggers for man-db ...

Setting up flex (2.5.35-9) ...

Because this package doesn't build anything. When I run the above command under KUbuntu 10.04, I get the following output:

.

the manual says:

Bison and Flex are required to build from Git, but not required when building from a release file. OnlyBison 1.875 or versions 2.2 and later will work. Flex must be version 2.5.31 or late