replacementString property Null safety

String used to replace banned patterns.

For deny lists (allow is false), each match of the filterPattern is replaced with this string. If filterPattern can match more than one character at a time, then this can result in multiple characters being replaced by a single instance of this replacementString.

For allow lists (allow is true), sequences between matches of filterPattern are replaced as one, regardless of the number of characters.

For example, consider a filterPattern consisting of just the letter "o", applied to text field whose initial value is the string "Into The Woods", with the replacementString set to *.

If allow is true, then the result will be "ooo*". Each sequence of characters not matching the pattern is replaced by its own single copy of the replacement string, regardless of how many characters are in that sequence.

If allow is false, then the result will be "Int* the W**ds". Every matching sequence is replaced, and each "o" matches the pattern separately.

If the pattern was the RegExp o+, the result would be the same in the case where allow is true, but in the case where allow is false, the result would be "Int* the W*ds" (with the two "o"s replaced by a single occurrence of the replacement string) because both of the "o"s would be matched simultaneously by the pattern.

The filter may adjust the selection and the composing region of the text after applying the text replacement, such that they still cover the same text. For instance, if the pattern was o+ and the last character "s" was selected: "Into The Wood|s|", then the result will be "Into The W*d|s|", with the selection still around the same character "s" despite that it is now the 12th character.

In the case where one end point of the selection (or the composing region) is strictly inside the banned pattern (for example, "Into The |Wo|ods"), that endpoint will be moved to the end of the replacement string (it will become "Into The |W*|ds" if the pattern was o+ and the original text and selection were "Into The |Wo|ods").