Manipulating first character of each line in a file with .Replace() - powershell

Question

Say I have a text file 123.txt
one,two,three
four,five,six
My goal is to capitalize the first character of each line by using Get-Culture. This is my attempt:
$str = gc C:\Users\Administrator\Desktop\123.txt
#Split each line into an array
$array = $str.split("`n")
for($i=0; $i -lt $array.Count; $i++) {
#Returns O and F:
$text = (Get-Culture).TextInfo.ToTitleCase($array[$i].Substring(0,1))
#Supposed to replace the first letter of each array with $text
$array[$i].Replace($array[$i].Substring(0,1), $text) >> .\Desktop\finish.txt
}
Result:
One,twO,three
Four,Five,six
I understand that .Replace() is replaces every occurrence of the current array, which is why I made sure that it's replacing ONLY the first character of the array with $array[$i].Substring(0,1), but this doesn't work.

Answer 1

Try the following:
Get-Content C:\Users\Administrator\Desktop\123.txt | ForEach-Object {
if ($_) {
$_.Substring(0, 1).ToUpper() + $_.Substring(1)
} else {
$_
}
} > .\Desktop\finish.txt
Get-Content reads the input file line by line and sends each line - stripped of its line terminator - through the pipeline.
ForEach-Object processes each line in the associated script block, in which $_ represents the line at hand:
if ($_) tests if the line is nonempty, i.e. if there's at least 1 character; if not, the else block simply passes the empty line through.
$_.Substring(0, 1).ToUpper() converts the line's 1st character to uppercase, implicitly using the current culture (with a single character, this is equivalent to applying Get-Culture).TextInfo.ToTitleCase()).
+ $_.Substring(1) appends the rest of the line.
Only > rater than >> is needed to write to the output file, because the entire pipeline's output is written at once.

Answer 2

The reason this is not working is because you are replacing the character...
$array[$i].Substring(0,1)
... but you are using the Replace method on the entire array element
$array[$i].Replace(...
Here the array element is a string, equal to a line of the input. So it will replace every occurrence of that character.
Get-Content (unless you use the -Raw parameter) by default returns the text as an array of strings. So you should be able to use this regex replace (I have used ToString().ToUpper() - nothing wrong with the Get-Culture method)
$str = gc C:\Users\Administrator\Desktop\123.txt
foreach($line in $str){
$line -replace '^\w', $line[0].ToString().ToUpper() >> .\Desktop\finish.txt
}
Regex explanation:
^ is an anchor. It specifies "the beginning of the string"
\w matches a word character - usually a-z, A-Z, 0-9
See mklement0's comments here for the more focused ^\p{Ll} and here for further explanation