I have a custom action list that replace various stuff such as:
FIELD: _ALL
On > on
Is > is
etc ....
my problem is some song titles have these words in the beginning of them so how can I use scripting functions to not have the the utmost left character be changed 
If the replace is targeted at single words within a string, you can simply add a leading blank to the words - at the beginning of a field there should not be a blank.
Give me an example?
one of my replace action rule is as follows:
http://i.imgur.com/5QM4jxb.jpg
what do I do?
You have to judge if the replacements are a word of their own or part of a word.
So, to replace the single "A" to "a" would mean that you look for " A " and replace it with " a ".
If the "a" is the start of a word (like "and") then you would search for " A" and replace it with " a".
Common to these examples is that the sought-after letter combination has a " " (=blank) in front - if the letter combination is found at the beginning of a field then there is no blank in front.
Clear enough?
I understand. The problem with MP3Tag "Replace" function is it doesn't recognize the space (character)
This doesn't work:
You have to uncheck [ ] only as whole word
Or instead of changing all your actions you could add one action that makes the first character upper case again.
INTERESTING! I can't come up with a code that will identify that though? here's what I have so far (which I believe is the most appropriate method):
So you problem is not to have some words in lower case but to have the first letter of a field in upper case?
For this see
/t/967/1
~Persay 
It's just I want to have only these following words lowercase unless it's not in the beginning of the field (most specifically %title%).
a|am|an|and|are|as|at|but|by|dj|mc|for|from|if|in|is|it|my|nor|of|or|on|the|to|s
o|yet|your
so i'm still in a slump about this. What you mentioned earlier is perhaps the best fit for me, it's just I don't think there is a case conversion specific code for only specific character modification.
To make the first letter upper case use
Action type: Replace with regular expression
Field: _ALL
Regular expression: ^(\l)
Replace matches with: $upper($1)