Question

I’m trying to learn regex in javascript. While doing testing, my mind get confused on seeing the output.

Can you please explain why my output result array has empty string while using * in pattern and not in +.

PS: The inputs taken are just a sample simple string to learn.

Here’s my sample code:

a='bb bbbb bbb'
a.match(/(bb)*/g) // O/P is  ["bb","","bbbb","","bb","",""]
a.match(/(bb)+/g) // O/P is ["bb","bbbb","bb"] 

Answer 1

The *, or Kleene Star, in regular expressions means “zero or more”, so it matches empty string.

The + means “one or more” instead, so it does not match empty string.

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Given that we have as follows

a = 'bb bbbb bbb';

Let’s represent it like this:

a = /^bb bbbb bbb$/;

where ^ is start of string and $ is end of string.

Our pattern is (bb)*. This means that the engine will look for a sequence of two b “zero or more times”. This means the pattern matches bb if the sequence is found or empty string otherwise.

Now step by step I’ll use a dot (.) to represent regexp analysis.

STEPS

        STRING              MATCH ARRAY
start - /^.bb bbbb bbb$/ -> [ ]
    1 - /^bb. bbbb bbb$/ -> [ "bb" ]
    2 - /^bb .bbbb bbb$/ -> [ "bb", "" ]
    3 - /^bb bbbb. bbb$/ -> [ "bb", "", "bbbb" ]
    4 - /^bb bbbb .bbb$/ -> [ "bb", "", "bbbb", "", ]
    5 - /^bb bbbb bb.b$/ -> [ "bb", "", "bbbb", "", "bb" ]
    6 - /^bb bbbb bbb.$/ -> [ "bb", "", "bbbb", "", "bb", "", ]
    7 - /^bb bbbb bbb$./ -> [ "bb", "", "bbbb", "", "bb", "", "" ]

In STEP 1 we have pushed bb since the pattern matched one sequence.

In STEP 2 we have pushed "" because the pattern matched empty string.

In STEP 3 we have pushed bbbb because the pattern matched two sequences.

STEP 4 -> STEP 2

STEP 5 -> STEP 1

In STEP 6 we have pushed "" because the pattern matched empty string since, only one b was found.

Finally you encounter $ token and it matches empty string again.

Note that if the string was "bb bbbb bbbb" the array would have been ["bb", "", "bbbb", "", "bbbb", ""]