名前付き文字参照

What Are Named Character References?

Named character references (also called named HTML entities) are predefined shorthand sequences that represent specific Unicode characters in HTML. They follow the pattern &name; where name is a case-sensitive keyword registered in the HTML specification. Examples include & for the ampersand character, < for the less-than sign, and © for the copyright symbol ©.

The HTML5 specification defines 2,231 named references, covering characters across many Unicode blocks: Latin letters with diacritics, Greek and Cyrillic letters, mathematical symbols, arrows, currency signs, playing card suits, and more.

History and Design

Named references were introduced early in HTML to help authors write documents using only ASCII source files while displaying richer character sets. Before UTF-8 became universal, a document might be saved in Latin-1 but still need to display Greek letters or typographic dashes. Named entities provided a portable solution.

HTML4 borrowed many entities from ISO character sets and SGML. HTML5 dramatically expanded the list by adding all characters from MathML and a large portion of common Unicode symbols.

The Five Essential Entities

Five named references are special because they escape HTML syntax characters:

&lt;    → <   (less-than, starts tags)
&gt;    → >   (greater-than, ends tags)
&amp;   → &   (ampersand, starts entities)
&quot;  → "   (double quote, used in attributes)
&apos;  → '   (apostrophe, HTML5 only; use &#39; for HTML4)

Forgetting to escape these — especially & and < in user-generated content — is the root cause of many XSS vulnerabilities.

Commonly Used Named References

<!-- Typography -->
&nbsp;    → non-breaking space (U+00A0)
&mdash;   → — (em dash, U+2014)
&ndash;   → – (en dash, U+2013)
&ldquo;   → " (left double quote)
&rdquo;   → " (right double quote)
&lsquo;   → ' (left single quote)
&rsquo;   → ' (right single quote, apostrophe)
&hellip;  → … (ellipsis)

<!-- Currency -->
&euro;    → € (U+20AC)
&pound;   → £ (U+00A3)
&yen;     → ¥ (U+00A5)
&cent;    → ¢ (U+00A2)

<!-- Math and science -->
&times;   → × (U+00D7)
&divide;  → ÷ (U+00F7)
&plusmn;  → ± (U+00B1)
&infin;   → ∞ (U+221E)
&sum;     → ∑ (U+2211)
&radic;   → √ (U+221A)
&pi;      → π (U+03C0)

<!-- Arrows -->
&larr;    → ← (U+2190)
&rarr;    → → (U+2192)
&uarr;    → ↑ (U+2191)
&darr;    → ↓ (U+2193)

Case Sensitivity

Named references are case-sensitive. © works, © and &Copy; do not. A few names have uppercase variants that mean something different: Α is Α (Greek capital Alpha), while α is α (Greek small alpha).

Verifying Named References

// Decode a named entity by injecting into DOM
function decodeEntity(entity) {
  const el = document.createElement("textarea");
  el.innerHTML = entity;
  return el.value;
}

decodeEntity("—");   // "—"
decodeEntity("…");  // "…"
decodeEntity("&notaref;"); // "&notaref;" — unknown entities are left as-is

When to Use Named References Today

With UTF-8 encoding, you can paste ©, —, and ∞ directly into HTML source. Named references are mainly useful for: code samples that must remain plain ASCII, generated HTML in environments where the encoding might shift, and developer readability when the intent matters (  is clearer than a bare invisible space character).

Quick Facts