Restricting Character length in Regular expression

Question

I am using the following regular expression without restricting any character length

var test =  /^(a-z|A-Z|0-9)*[^$%^&*;:,<>?()\""\']*$/ //Works Fine

In above when I am trying to restrict the characters length to 15 as below, it throws an error.

var test =  /^(a-z|A-Z|0-9)*[^$%^&*;:,<>?()\""\']*${1,15}/    //**Uncaught SyntaxError: Invalid regular expression**

Please help me to make the above regEx work with the characters limit to 15.

Answer 1

You cannot apply quantifiers to anchors. Instead, to restrict the length of the input string, use a lookahead anchored at the beginning:

^(?=.{1,15}$)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*$
  ^^^^^^^^^^^

Also, I assume you wanted to match 0 or more letters or digits with (a-z|A-Z|0-9)*. It should look like [a-zA-Z0-9]* (i.e. use a character class here).

Why not use a limiting quantifier, like {1,15}, at the end?

Quantifiers are only applied to the subpattern to the left, be it a group or a character class, or a literal symbol. Thus, ^[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']{1,15}$ will effectively restrict the length of the second character class [^$%^&*;:,<>?()\"'] to 1 to 15 characters. The ^(?:[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*){1,15}$ will "restrict" the sequence of 2 subpatterns of unlimited length (as the * (and +, too) can match unlimited number of characters) to 1 to 15 times, and we still do not restrict the length of the whole input string.

How does the lookahead restriction work?

The (?=.{1,15}$) positive lookahead appears right after ^ start-of-string anchor. It is a zero-width assertion that only returns true or false after checking if its subpattern matches the subsequent characters. So, this lookahead tries to match any 1 to 15 (due to the limiting quantifier {1,15}) characters but a newline right at the end of the string (due to the $ anchor). If we remove the $ anchor from the lookahead, the lookahead will only require the string to contain 1 to 15 characters, but the total string length can be any.

If the input string can contain a newline sequence, you should use [\s\S] portable any-character regex construct (it will work in JS and other common regex flavors):

^(?=[\s\S]{1,15}$)[a-zA-Z0-9]*[^$%^&*;:,<>?()\"']*$
 ^^^^^^^^^^^^^^^^^