[nerv_]

var re = /\b0+\B/g;

alert( "1010+2020".replace( re, "" ) );
alert( "010-020+030".replace( re, "" ) );
alert( "010*020".replace( re, "" ) );
alert( "010 / 020".replace( re, "" ) );
alert( "010 + 020".replace( re, "" ) );
alert( "000010 - 0000020".replace( re, "" ) );

alert( "0 - 0".replace( re, "" ) );

alert( "из числа 000010 вычесть 0000020 получим -00000010".replace( re, "" ) );