monit install problem

I was getting error message while ./configure of monit:

checking for gcc... gcc
checking for C compiler default output file name... a.out
checking whether the C compiler works... yes
checking whether we are cross compiling... no
checking for suffix of executables...
checking for suffix of object files... o
checking whether we are using the GNU C compiler... yes
checking whether gcc accepts -g... yes
checking for gcc option to accept ISO C89... none needed
checking how to run the C preprocessor... gcc -E
checking for grep that handles long lines and -e... /bin/grep
checking for egrep... /bin/grep -E
checking whether gcc needs -traditional... no
checking for a BSD-compatible install... /usr/bin/install -c
checking whether make sets $(MAKE)... yes
checking for flex... no
checking for lex... no
checking for yywrap in -lfl... no
checking for yywrap in -ll... no
configure: error: monit requires flex, not lex

Flex was not installed. I installed that via:

apt-get install flex

After doing that, ./configure works fine.