console.log(`original solution ${C:\backup\.replace(/\\/g, '\')}`)



// a template literal will automagically replace \ with 

console.log(`template string without further ado ${`C:\backup\`}`);



// but if they are escaped themselves

console.log(`Double escaped ${`C:\\backup\\`.replace(/\{2,}/g, '\')}`);



// multiple escaped

console.log(`multiple escaped ${`C:\\\\backup\\`

  .replace(/\{2,}/g, '\')}`);



// don't want to replace the last \

console.log(`not the last ${`C:\\\backup\\`

  .replace(/\{2,}([^\{2,}$])/g, (a ,b) => a.slice(0,1) + b)}` );



// don't want to replace the first \

console.log(`not the first ${`C:\\backup\`.replace(/\[^\]$/g, '\')}`);



// a generic tagged template to replace all multiple \ OR //

const toSingleSlashes = (strs, ...args) => 

  strs.reduce( (a, v, i ) => 

    a.concat(args[i-1] || ``).concat(v, ``), `` )

      .replace( /(\|/){2,}/g, (a, b) => b );



console.log(`generic tagged template 1 ${

  toSingleSlashes`C:\backup\`}`);



console.log(`generic tagged template 2 ${

  toSingleSlashes`C:\\\\backup\\\\\`}`);

  

console.log(`generic tagged template 3 ${

  toSingleSlashes`C:\\\\backup\`}`);

  

console.log(`generic tagged template 4 ${

  toSingleSlashes`C:////////backup////`}`);



console.log(`reply to comment @chitgoks => 

test\\hehehe is by default test\hehehe

so, no replacement necessary here ...`);

.as-console-wrapper {

    max-height: 100% !important;

}