Matches quantities dimensioned by SI units in typical markdown-formatted plaintext documents. Replaces any spaces or HTML entities between the number and its dimensions with a "narrow non-breaking space" character. This character may fallback to a "thin space" due to its font support. The lexicographical convention for SI units is to use a full space, not a narrow space, but that was not the concern for this use case. This was submitted under PCRE by accident—it is for ECMAScript.