How can I view log files in Linux and apply custom filters while viewing?

Use &pattern command within less.

From the man page for less

&pattern

          Display  only  lines which match the pattern; lines which do not
          match the pattern are not displayed.  If pattern  is  empty  (if
          you  type  &  immediately  followed  by ENTER), any filtering is
          turned off, and all lines are displayed.  While filtering is  in
          effect,  an  ampersand  is  displayed  at  the  beginning of the
          prompt, as a reminder that some lines in the file may be hidden.

          Certain characters are special as in the / command:

          ^N or !
                 Display only lines which do NOT match the pattern.

          ^R     Don't interpret regular expression  metacharacters;  that
                 is, do a simple textual comparison.

Try the multitail tool - as well as letting you view multile logs at once, I'm pretty sure it lets you apply regex filters interactively.

Based on ghostdog74's answer and the less manpage, I came up with this:

~/.bashrc:

export LESSOPEN='|~/less-filter.sh %s'
export LESS=-R  # to allow ANSI colors

~/less-filter.sh:

#!/bin/sh
case "$1" in
*logfile*.log*) ~/less-filter.sed < $1
  ;;
esac

~/less-filter.sed:

/deleteLinesLikeThis/d  # to filter out lines
s/this/that/  # to change text on lines (useful to colorize using ANSI escapes)

Then:

• less logfileFooBar.log.1 -- applies the filter applies automatically.
• cat logfileFooBar.log.1 | less -- to see the log without filtering

This is adequate for now but I would still like to be able to edit the filters on the fly.