Match word by its prefix - regex

Question

I'm trying to match a string by its prefix that ends with a particular character. For example, if my string is "abcd" and ends in #, then any word which is a prefix of "abcd" should be matched as long as it ends with #. Here are some examples to help illustrate the pattern:
Input: "ab#" gives true (as "ab" is a prefix of "abcd" and end with a #).
Input: "abcd#" gives true (as "abcd" is a prefix of "abcd" and end with a #).
Input: "bc#" gives false (as "bc" is a not a prefix of "abcd" ).
Input: "ab#" gives false (while "ab" is a prefix of "abcd", it doesn't end with #) .
Input: "ac#" gives false (while "ac" is contained within "abcd", it doesn't begin with a prefix from "abcd") .
So far, I've managed to come up with the following expression which seems to be working fine:
/(abcd|abc|ab|a)#/
While this is working, it isn't very practical, as larger words of length n will make the expression quite large:
/(n|n-1|n-2| ... |1)#/
Is there a way to rewrite this expression so it is more scalable and concise?
Example of my attempt (in JS):
const regex = /(abcd|abc|ab|a)#/;
console.log(regex.test("abcd#")); // true
console.log(regex.test("ab#")); // true
console.log(regex.test("abc#")); // true
console.log(regex.test("abz#")); // false
console.log(regex.test("abc#")); // false
Edit: Some of the solutions provided are nice and do do what I'm after, however, for this particular question, I'm after a solution which uses pure regular expressions to match the prefix.

Answer 1

Just use String#startsWith and String#endsWith here:
String input = "abcd";
String prefix = "ab#";
if (input.startsWith(prefix.replaceAll("#$", "")) && prefix.endsWith("#")) {
System.out.println("MATCH");
}
else {
System.out.println("NO MATCH");
}
Edit: A JavaScript version of the above:
var input = "abcd";
var prefix = "ab#";
if (input.startsWith(prefix.replace(/#$/, "")) && prefix.endsWith("#")) {
console.log("MATCH");
}
else {
console.log("NO MATCH");
}

Answer 2

Try ^ab?c?d?#$
Explanation:
`^` - match beginning of a string
`b?` - match match zero or one `b`
Rest is analigocal to the above.
Demo

Answer 3

Here's a left field JavaScript option. Build and array of valid prefixes, use join on the array to make your regex pattern.
var validPrefixes = ["abcd",
"abc",
"ab",
"a",
"areallylongprefix"];
var regexp = new RegExp("^(" + validPrefixes.join("|") + ")#$");
console.log(regexp.test("abcd#"));// true
console.log(regexp.test("ab#")); // true
console.log(regexp.test("abc#")); // true
console.log(regexp.test("abz#")); // false
console.log(regexp.test("abc#")); // false
console.log(regexp.test("areallylongprefix#")); //true
This can be adapted to the language of tour choosing, also handy if your prefixes are dynamically retrieved from a database or similar.

Answer 4

Here's my c# attempt:
private static bool test(string v)
{
var pattern = "abcd#";
//No error handling
return v.EndsWith(pattern[pattern.Length-1])
&& pattern.Replace("#", "").StartsWith(v.Replace("#",""));
}
Console.WriteLine(test("abcd#")); // true
Console.WriteLine(test("ab#")); // true
Console.WriteLine(test("abc#")); // true
Console.WriteLine(test("abz#")); // false
Console.WriteLine(test("abc#")); // false
Console.WriteLine(test("abc")); //false

Answer 5

/a(b(cd?)?)?#/
Or for a longer example, to match a prefix of "abcdefg#":
/a(b(c(d(e(fg?)?)?)?)?)?#/
Generating this regex isn't completely trivial, but some options are:
function createPrefixRegex(s) {
// This method creates an unnecessary set of parentheses
// around the last letter, but that won't harm anything.
return new RegExp(s.slice(0,-1).split('').join('(') + ')?'.repeat(s.length - 2) + '#');
}
function createPrefixRegex2(s) {
var r = s[0];
for (var i = 1; i < s.length - 2; ++i) {
r += '(' + s[i];
}
r += s[s.length - 2] + '?' + ')?'.repeat(s.length - 3) + '#';
return new RegExp(r);
}
function createPrefixRegex3(s) {
var recurse = function(i) {
if (i >= s.length - 1) {
return '';
}
if (i === s.length - 2) {
return s[i] + '?';
}
return '(' + s[i] + recurse(i + 1) + ')?';
}
return new RegExp(s[0] + recurse(1) + '#');
}
These may fail if the input string has no prefix before the '#' character, and they assume the last character in the string is '#'.