Why can't "[[:space:]]" match Line Feed(LF) on grep command?

Solution Verified - Updated -

Issue

  • Why can't "[[:space:]]" match Line Feed(LF) on grep command?

    # cat /etc/hosts
# Do not remove the following line, or various programs
# that require network functionality will fail.
127.0.0.1               localhost.localdomain localhost
::1             localhost6.localdomain6 localhost6

# grep "[[:space:]]localhost" /etc/hosts 

127.0.0.1              localhost.localdomain localhost
::1            localhost6.localdomain6 localhost6
# grep "[[:space:]]localhost[[:space:]]" /etc/hosts
#

  • I expect the following. Because "[[:space:]]" includes LF.

    # grep "[[:space:]]localhost[[:space:]]" /etc/hosts
127.0.0.1               localhost.localdomain localhost

Environment

  • Red Hat Enterprise Linux 4

  • Red Hat Enterprise Linux 5

  • Red Hat Enterprise Linux 6

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