function reDurationString() {
	return /^(\d+d)?(\d+h)?(\d+m)?(\d+s)?(\d+ms)?$/i;
}
Returns a regular expression to match a duration string.

## Notes

-   A duration string is a string containing a sequence of time units. A time unit is a nonnegative integer followed by a unit identifier. The following unit identifiers are supported:

    -   `d`: days
    -   `h`: hours
    -   `m`: minutes
    -   `s`: seconds
    -   `ms`: milliseconds

    For example, the string `1m3s10ms` is a duration string containing three time units: `1m` (1 minute), `3s` (3 seconds), and `10ms` (10 milliseconds). The string `60m` is a duration string containing a single time unit: `60m` (60 minutes). Time units must be supplied in descending order of magnitude (i.e., days, hours, minutes, seconds, milliseconds).

-   Duration strings are case insensitive. For example, the string `1M3S10MS` is equivalent to `1m3s10ms`.

-   The regular expression captures the following groups:

    1.  The days component.
    2.  The hours component.
    3.  The minutes component.
    4.  The seconds component.
    5.  The milliseconds component.

@returns {RegExp} regular expression

@example
var RE_DURATION = reDurationString();
// returns <RegExp>