Use perl instead of sed with GNU-only "/I" feature

#23

README.org

@@ -54,18 +54,14 @@ As a bonus:
   documentation are generated using Org Babel from [[https://github.com/kaushalmodi/eless/blob/master/eless.org][one file]] (even this
   README).
 * Requirements
-- Emacs 24.4 or newer is needed if you want to build =eless= locally
-  from =eless.org=.
-  - If you plan to just download the use the =eless= script, it will
-    run on older emacs versions too.
-- This is a =bash= script. So even if you don't use =bash= shell, you
-  need to have the =bash= binary discoverable through your environment
-  variable =PATH=.
-  - Tested to work in =tcsh= shell (/I know/) on RHEL 6.6.
-- *GNU =grep=* for =-P= flag (Perl regex) support
-  - Tested to work with GNU =grep= 2.6.3 on RHEL 6.6.
-  - Mac OS users would need to install GNU =grep= [[https://apple.stackexchange.com/a/193300][using =brew
-        install=]].
+|------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Software   | Details                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         |
+|------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| =emacs=    | If *developing* (when running =make eless= or =make docs=), the minimum required version is 24.4. If only *running* the =eless= script, older emacs versions should work fine (sparsely tested). So if you face a problem, [[https://github.com/kaushalmodi/eless/issues][report it]], following the *How to help debug* section in [[https://github.com/kaushalmodi/eless/blob/master/CONTRIBUTING.org][CONTRIBUTING]]. /Both building and running of =eless= tested on emacs 24.4, RHEL 6.6./ |
+| =bash=     | This is a =bash= script. So even if you don't use =bash= shell, you need to have the =bash= binary discoverable through your environment variable =PATH=. /Tested to work in =tcsh= shell on RHEL 6.6./                                                                                                                                                                                                                                                                                         |
+| GNU =grep= | *GNU* =grep= is required for the =-P= flag support (for Perl regex). /macOS/ users would need to install GNU =grep= [[https://apple.stackexchange.com/a/193300][using =brew install=]]. /Tested to work with GNU =grep= 2.6.3 on RHEL 6.6./                                                                                                                                                                                                                                                     |
+| =perl=     | Perl is used to replace case-insensitive =sed= based replacements (using =/I=) as that feature is available only in GNU =sed=. Perl would be available on more systems compared to GNU =coreutils=. /Tested with Perl v5.16.3 on RHEL 6.6./                                                                                                                                                                                                                                                     |
+|------------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
 * Try it out
 Here are some usage examples:
 #+BEGIN_SRC shell