PHP string word last character case exchange, string case

requirement

Given a string such as "A journey of, a thousand 'miles' must can't "begin" with a single step.", it will be processed by the PHP program into "a journeY oF, A thousanD 'mileS' musT can'T "begiN" with A singlE steP."

Note:
1. If the last character of each word is uppercase, it will become lowercase. If it is lowercase, it will become uppercase.
2. You need to consider conversions in the form of can't.
3. Punctuation (only , ' " . ;) does not need to be changed.

Reference Algorithm

<?<span>php

    </span><span>function</span> convertLastChar(<span>$str</span><span>) {
        </span><span>$markArr</span> = <span>array</span>(", ", "' ", "\" ", ". ", "; "<span>);
        </span><span>$ret</span> = ""<span>;
        </span><span>for</span> (<span>$i</span> = 0, <span>$j</span> = <span>strlen</span>(<span>$str</span>); <span>$i</span> < <span>$j</span>; <span>$i</span>++<span>) {
            </span><span>if</span> (<span>$i</span> < <span>$j</span> - 2<span>) {
                </span><span>$afterStr</span> = <span>$str</span>{<span>$i</span> + 1} . <span>$str</span>{<span>$i</span> + 2<span>};
            } </span><span>else</span> <span>if</span> (<span>$i</span> < <span>$j</span> - 1<span>) {
                </span><span>$afterStr</span> = <span>$str</span>{<span>$i</span> + 1} . " "<span>;
            }
            </span><span>if</span> (<span>in_array</span>(<span>$afterStr</span>, <span>$markArr</span><span>) 
                </span>|| <span>$i</span> == <span>$j</span> - 1 
                || <span>$str</span>{<span>$i</span> + 1} == " "<span>) {
                </span><span>$ret</span> .= <span>strtoupper</span>(<span>$str</span>{<span>$i</span>}) === <span>$str</span>{<span>$i</span><span>} 
                    </span>? <span>strtolower</span>(<span>$str</span>{<span>$i</span><span>}) 
                    </span>: <span>strtoupper</span>(<span>$str</span>{<span>$i</span><span>});
            } </span><span>else</span><span> {
                </span><span>$ret</span> .= <span>$str</span>{<span>$i</span><span>};
            }
        }
        </span><span>return</span> <span>$ret</span><span>;
    }

</span>?>

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Test

<?<span>php

    </span><span>//</span><span>test</span>
    <span>$str1</span> = "A journey of, a thousand 'miles' must can't \"begin\" with a single step."<span>;
    </span><span>$str2</span> = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. "<span>;
    </span><span>$str3</span> = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a "<span>;
    </span><span>$str4</span> = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a B"<span>;
    </span><span>$str5</span> = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a b'"<span>;
    </span><span>$str6</span> = "A journey of, a thousand 'miles' must can't \"begin\" with a single step. a B\""<span>;

    </span><span>echo</span> "source:<br/>" . <span>$str1</span> . "<br/>result:<br/>" . convertLastChar(<span>$str1</span>) . "<br/><br/>"<span>;
    </span><span>echo</span> "source:<br/>" . <span>$str2</span> . "<br/>result:<br/>" . convertLastChar(<span>$str2</span>) . "<br/><br/>"<span>;
    </span><span>echo</span> "source:<br/>" . <span>$str3</span> . "<br/>result:<br/>" . convertLastChar(<span>$str3</span>) . "<br/><br/>"<span>;
    </span><span>echo</span> "source:<br/>" . <span>$str4</span> . "<br/>result:<br/>" . convertLastChar(<span>$str4</span>) . "<br/><br/>"<span>;
    </span><span>echo</span> "source:<br/>" . <span>$str5</span> . "<br/>result:<br/>" . convertLastChar(<span>$str5</span>) . "<br/><br/>"<span>;
    </span><span>echo</span> "source:<br/>" . <span>$str6</span> . "<br/>result:<br/>" . convertLastChar(<span>$str6</span>) . "<br/><br/>"<span>;
</span>?>

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Result:

source:<span>
A journey of</span>, a thousand 'miles' must can'<span>t "begin" with a single step.
result:
a journeY oF, A thousanD </span>'mileS' musT can'T "begiN" witH A singlE steP.<span>

source</span>:<span>
A journey of</span>, a thousand 'miles' must can'<span>t "begin" with a single step. 
result:
a journeY oF, A thousanD </span>'mileS' musT can'T "begiN" witH A singlE steP.<span> 

source</span>:<span>
A journey of</span>, a thousand 'miles' must can'<span>t "begin" with a single step. a 
result:
a journeY oF, A thousanD </span>'mileS' musT can'T "begiN" witH A singlE steP.<span> A 

source</span>:<span>
A journey of</span>, a thousand 'miles' must can'<span>t "begin" with a single step. a B
result:
a journeY oF, A thousanD </span>'mileS' musT can'T "begiN" witH A singlE steP.<span> A b

source</span>:<span>
A journey of</span>, a thousand 'miles' must can't "begin" with a single step. a b'<span>
result</span>:<span>
a journeY oF</span>, A thousanD 'mileS' musT can'T "begiN" witH A singlE steP. A B'<span>

source</span>:<span>
A journey of</span>, a thousand 'miles' must can'<span>t "begin" with a single step. a B"
result:
a journeY oF, A thousanD </span>'mileS' musT can'T "begiN" witH A singlE steP. A b"<span><br /></span>

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We can see that it is in line with expectations.