Regex to match middle x digits in a number-CodePudding

Need some help with a regex to match middle x number of digits in javascript ie.

Numbers

123456 3234231234

3234231234

Expected Outcome x=3

123456 323***1234

323***1234

The mask * length can vary ie 3 -> ***
If space is present it should select the middle numbers in the second set

So far what I have is /(?<=\s).{3}/g which just gets the first few digits and does not work when space is not present, this is how it shows up,

123456 ***4231234 *(first 3 digits)*

42342341234 *(does not match as required above)*

CodePudding user response：

Because you're dealing with a small set of possible widths, you can develop a sub-expression for each size and or them together. One possible expression for only odd lengths might be:

(?:.* )?\b(?:(.{3})|.(.{3}).|..(.{3})..|...(.{3})...|....(.{3})....)\b

Although it will double the size, it shouldn't be difficult to expand the expression to include even lengths as well.

CodePudding user response：

I dont think regex are the right tool for the 100% of the job

You can use it to segment and operate your string, and then use string functions to replace the parts you need with *

check this out

function mask(value, digits, masker = "*"){
    // divides the string if has a space
    let components = value.match(/(\d*?\s?)(\d )$/);
    // only will mask the last part
    let target = components[2];
    let leading = components[1];
    // calculate the middle of the string and the part to replace
    let length = target.length;
    let lengthBeforeMask = Math.floor((length/2)-(digits/2));
    let lengthAfterMask = Math.ceil((length/2)-(digits/2));
    // use templates to concatenate the string and substring to replace the parts with the mask
    return `${leading}${target.substring(0,lengthBeforeMask)}${masker.repeat(digits)}${target.substring(length - lengthAfterMask, length)}`;

}

// use it
mask("123456 3234231234", 3); // "123456 323***1234"
mask("3234231234", 3); // "323***1234"
mask("3234231234", 5); // "32*****234"
// also can change the mask just ;)
mask("12345", 3, "&"); // "1&&&5"