One option is to use a Masked Textbox.
In your example, you would set the mask to:
"The quick brown fox jLLLed over the lazy hound"
Which would appear as:
"The quick brown fox j___ed over the lazy hound"
And only allow 3 characters (a-z & A-Z) to be entered into the gap.
And the mask could be easily changed via code.
EDIT:
For convenience...
Here is a list and description of masking characters
(taken from http://www.c-sharpcorner.com/uploadfile/mahesh/maskedtextbox-in-C-Sharp/).
0 - Digit, required. Value between 0 and 9.
9 - Digit or space, optional.
# - Digit or space, optional. If this position is blank in the mask, it will be rendered as a space in the Text property.
L - Letter, required. Restricts input to the ASCII letters a-z and A-Z.
? - Letter, optional. Restricts input to the ASCII letters a-z and A-Z.
& - Character, required.
C - Character, optional. Any non-control character.
A - Alphanumeric, required.
a - Alphanumeric, optional.
. - Decimal placeholder.
, - Thousands placeholder.
: - Time separator.
/ - Date separator.
$ - Currency symbol.
< - Shift down. Converts all characters that follow to lowercase.
> - Shift up. Converts all characters that follow to uppercase.
| - Disable a previous shift up or shift down.
- Escape. Escapes a mask character, turning it into a literal. "" is the escape sequence for a backslash.
All other characters - Literals. All non-mask elements will appear as themselves within MaskedTextBox. Literals always occupy a static position in the mask at run time, and cannot be moved or deleted by the user.
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