Appending the matched text like that won't work, you need to do it in two steps:

/<regex>/;
$amourss.=$1;
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This is what I'm guessing you want to do with your regular expression(with comments):

/^    # Match the start of the string
 a    # Single occurrence of the character a
 (.+) # Grab one or more other characters
      # and put into $1
 [fg] # a single character, either "f" or "g"
 $    # match the end of the string
/x    # x modifier to allow comments in regex
[download]

So your complete code would look something like this:

/^a(.+)[fg]$/
$amourss.=$1;
[download]