正则表达式：字符串以3到4个连续的“ a”结尾

Question

Write a regexp that matches string that finish with 3 to maximally 4 consecutive "a".

Tried the following and tests/outputs below. They're supposed to all print "good". What am I missing here?

Source: Codecademy

//find the regexp to 
//all the words that finish with a minimum of 3 to a maximum of 4 consecutive "a"
var threeToFourConsecutiveA = /^[a]+a{3,4}$/; // THIS IS WHAT I DID

//when you run it, the code should only write down "good" messages 
console.log(threeToFourConsecutiveA.test("baa")?"bad":"good"); 
console.log(threeToFourConsecutiveA.test("baaa")?"good":"bad"); 
console.log(threeToFourConsecutiveA.test("caaaa")?"good":"bad");
console.log(threeToFourConsecutiveA.test("daaaaa")?"bad":"good");
console.log(threeToFourConsecutiveA.test("aaz")?"bad":"good");
console.log(threeToFourConsecutiveA.test("aaaz")?"bad":"good");
console.log(threeToFourConsecutiveA.test("aaaaz")?"bad":"good");
console.log(threeToFourConsecutiveA.test("azazaza")?"bad":"good");

Answer 1

In regular expressions, the caret ^ has two different meanings:

• Inside the brackets, it means any character that is not from the following .
• Outside the brackets, it means the string must start here .

Compare these two:

/^[a]+a{3,4}$/ (wrong)
/[^a]+a{3,4}$/ (correct)

(The above explanation is not a 100 percent accurate, as language lawyers will happily point out, but for now it suffices.)

Answer 2

You used "starts with one or more a" ^[a]+

Solution: (test it here )

var reg = /[^a]+a{3,4}$/;

---

Answer 3

Since [a] means a literal "a" your pattern can't be the good one.

Try:

^(?:[^a]+a+)*[^a]+a?a{3}$

Answer 4

Try this:

var threeToFourConsecutiveA = /^[^a]+a{3,4}$/;

I think you were missing the second ^ which excludes a .

Answer 5

Description

^[^a].*[b-z]a{3,4}$

This expression does the following

• validates words that start with anything but an "a", and finish with 3 to 4 consecutive "a".
• the [bz] forces a the last characters to be only 3 or 4 a 's. by using [az] or .* constructs these could allow the letter a to appear multiple times.

Example

Live Demo

https://regex101.com/r/iF8sN0/3

Sample text

bafaa
bafaaa
bafaaaa
bafaaaaa

Sample Matches

bafaaa
bafaaaa

Explanation

NODE                     EXPLANATION
----------------------------------------------------------------------
  ^                        the beginning of a "line"
----------------------------------------------------------------------
  [^a]                     any character except: 'a'
----------------------------------------------------------------------
  .*                       any character except \n (0 or more times
                           (matching the most amount possible))
----------------------------------------------------------------------
  [b-z]                    any character of: 'b' to 'z'
----------------------------------------------------------------------
  a{3,4}                   'a' (between 3 and 4 times (matching the
                           most amount possible))
----------------------------------------------------------------------
  $                        before an optional \n, and the end of a
                           "line"
----------------------------------------------------------------------

Alternative

^[^a][a-z]*a{3}$

Will allow any the last 3 or more characters to be a .