I'm trying to write a regex in which I want to test for 2 things. The string is only valid when it starts with 20 and its length is between 1 and 5, or when it doesn't start with 20 but it still has the plus sign as its first character (example 50) and its length is between 1 and 7 then it's valid too. Here is what I wrote for this validation :

/^\ (?=20){1,5}|^\ (?!20){1,7}/g

I tested it but it doesn't work how I intended for it and I dont understand why.

CodePudding user response：

What you have is close, but you need to:

• add the character selectors . to be quantified (. can work to match everything)
• constrain the end of the string to prevent the length from going over the max with $
• can remove g flag since you only want one match

That leaves you with:

/^\ (?=20).{1,5}$|^\ (?!20).{1,7}$/

which you can also write as

/^\ ((?=20).{1,5}|(?!20).{1,7})$/

CodePudding user response：

You can attempt to match the following regular expression:

^\ (?:20(\D.?)?|(?!20(?!\d)).{0,6})$

Demo

The expression can be broken down as follows.

^           # match beginning of string
\           # match ' '
(?:         # begin non-capture group
  20        # match '20'
  (?:\D.?)? # optionally match a non-digit that is optionally
            # followed by any character
|           # or
  (?!       # begin negative lookahead
    20      # match '20'
    (?!\d)  # negative lookahead asserts '20' is not followed by a digit
  )         # end negative lookahead
  .{0,6}    # match between 0 and 6 characters
)           # end non-capture group
$           # match end of string

Note that ' 200' is matched by the second part of the alternation (the string does not begin with '20').