tamzinblake/regex_end_of_string.md

## regex_end_of_string.md

      
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              regex_end_of_string.md
            
          
    This gist regards some confusion resulting from this StackOverflow question.
Refer to sections 15.5.4.10 and 15.5.4.11 of the spec - page 145ish.
Before any passes, the cursor is at the start of the string:
, " , f , o , o ,   , b , a , r , " ,
^

lastIndex = 0
thisIndex = 0
previousLastIndex = undefined
n = 0
A = []


After matching the start of string quote, the cursor is here:
, " , f , o , o ,   , b , a , r , " ,
    ^

lastIndex = 0
thisIndex = 1
previousLastIndex = 0
n = 1
A = ['"']


After matching the end of string quote, the cursor is here:
, " , f , o , o ,   , b , a , r , " ,
                                    ^

lastIndex = 9
thisIndex = 9
previousLastIndex = 0
n = 2
A = ['"', '"']


At this point, previousLastIndex does not equal thisIndex, so we try
to match again from the cursor position, successfully matching '0
double-quotes followed by the end of string'.  After that, the cursor
is here:
, " , f , o , o ,   , b , a , r , " ,
                                    ^

lastIndex = 9
thisIndex = 9
previousLastIndex = 9
n = 3
A = ['"', '"', '']


Since previousLastIndex == thisIndex, we set lastIndex to thisIndex+1,
which makes the regex fall off the end of the string.
And so the replacement replaces our three strings, ['"', '"', ''] with
'"', thus inserting one double-quote at the beginning and two at the
end.