aricb HTML character entity references are defined as part of the HTML specification: http://www.w3.org/TR/REC-html40/sgml/entities.html
The following code defines two commands, htmlentities::insertentities and htmlentities::removeentities. The first command replaces Unicode characters with HTML character entity references; the second command replaces HTML character entity references with the literal Unicode characters they represent. There's nothing fancy about the code, but hopefully it's useful to somebody.
namespace eval htmlentities {
variable map [list \
\u00a0  \; \
\u00a1 ¡\; \
\u00a2 ¢\; \
\u00a3 £\; \
\u00a4 ¤\; \
\u00a5 ¥\; \
\u00a6 ¦\; \
\u00a7 §\; \
\u00a8 ¨\; \
\u00a9 ©\; \
\u00aa ª\; \
\u00ab «\; \
\u00ac ¬\; \
\u00ad ­\; \
\u00ae ®\; \
\u00af ¯\; \
\u00b0 °\; \
\u00b1 ±\; \
\u00b2 ²\; \
\u00b3 ³\; \
\u00b4 ´\; \
\u00b5 µ\; \
\u00b6 ¶\; \
\u00b7 ·\; \
\u00b8 ¸\; \
\u00b9 ¹\; \
\u00ba º\; \
\u00bb »\; \
\u00bc ¼\; \
\u00bd ½\; \
\u00be ¾\; \
\u00bf ¿\; \
\u00c0 À\; \
\u00c1 Á\; \
\u00c2 Â\; \
\u00c3 Ã\; \
\u00c4 Ä\; \
\u00c5 Å\; \
\u00c6 Æ\; \
\u00c7 Ç\; \
\u00c8 È\; \
\u00c9 É\; \
\u00ca Ê\; \
\u00cb Ë\; \
\u00cc Ì\; \
\u00cd Í\; \
\u00ce Î\; \
\u00cf Ï\; \
\u00d0 Ð\; \
\u00d1 Ñ\; \
\u00d2 Ò\; \
\u00d3 Ó\; \
\u00d4 Ô\; \
\u00d5 Õ\; \
\u00d6 Ö\; \
\u00d7 ×\; \
\u00d8 Ø\; \
\u00d9 Ù\; \
\u00da Ú\; \
\u00db Û\; \
\u00dc Ü\; \
\u00dd Ý\; \
\u00de Þ\; \
\u00df ß\; \
\u00e0 à\; \
\u00e1 á\; \
\u00e2 â\; \
\u00e3 ã\; \
\u00e4 ä\; \
\u00e5 å\; \
\u00e6 æ\; \
\u00e7 ç\; \
\u00e8 è\; \
\u00e9 é\; \
\u00ea ê\; \
\u00eb ë\; \
\u00ec ì\; \
\u00ed í\; \
\u00ee î\; \
\u00ef ï\; \
\u00f0 ð\; \
\u00f1 ñ\; \
\u00f2 ò\; \
\u00f3 ó\; \
\u00f4 ô\; \
\u00f5 õ\; \
\u00f6 ö\; \
\u00f7 ÷\; \
\u00f8 ø\; \
\u00f9 ù\; \
\u00fa ú\; \
\u00fb û\; \
\u00fc ü\; \
\u00fd ý\; \
\u00fe þ\; \
\u00ff ÿ\; \
\u0192 &fnof\; \
\u0391 &Alpha\; \
\u0392 &Beta\; \
\u0393 &Gamma\; \
\u0394 &Delta\; \
\u0395 &Epsilon\; \
\u0396 &Zeta\; \
\u0397 &Eta\; \
\u0398 &Theta\; \
\u0399 &Iota\; \
\u039a &Kappa\; \
\u039b &Lambda\; \
\u039c &Mu\; \
\u039d &Nu\; \
\u039e &Xi\; \
\u039f &Omicron\; \
\u03a0 &Pi\; \
\u03a1 &Rho\; \
\u03a3 &Sigma\; \
\u03a4 &Tau\; \
\u03a5 &Upsilon\; \
\u03a6 &Phi\; \
\u03a7 &Chi\; \
\u03a8 &Psi\; \
\u03a9 &Omega\; \
\u03b1 &alpha\; \
\u03b2 &beta\; \
\u03b3 &gamma\; \
\u03b4 &delta\; \
\u03b5 &epsilon\; \
\u03b6 &zeta\; \
\u03b7 &eta\; \
\u03b8 &theta\; \
\u03b9 &iota\; \
\u03ba &kappa\; \
\u03bb &lambda\; \
\u03bc &mu\; \
\u03bd &nu\; \
\u03be &xi\; \
\u03bf &omicron\; \
\u03c0 &pi\; \
\u03c1 &rho\; \
\u03c2 &sigmaf\; \
\u03c3 &sigma\; \
\u03c4 &tau\; \
\u03c5 &upsilon\; \
\u03c6 &phi\; \
\u03c7 &chi\; \
\u03c8 &psi\; \
\u03c9 &omega\; \
\u03d1 &thetasym\; \
\u03d2 &upsih\; \
\u03d6 &piv\; \
\u2022 &bull\; \
\u2026 &hellip\; \
\u2032 &prime\; \
\u2033 &Prime\; \
\u203e &oline\; \
\u2044 &frasl\; \
\u2118 &weierp\; \
\u2111 &image\; \
\u211c &real\; \
\u2122 &trade\; \
\u2135 &alefsym\; \
\u2190 &larr\; \
\u2191 &uarr\; \
\u2192 &rarr\; \
\u2193 &darr\; \
\u2194 &harr\; \
\u21b5 &crarr\; \
\u21d0 &lArr\; \
\u21d1 &uArr\; \
\u21d2 &rArr\; \
\u21d3 &dArr\; \
\u21d4 &hArr\; \
\u2200 &forall\; \
\u2202 &part\; \
\u2203 &exist\; \
\u2205 &empty\; \
\u2207 &nabla\; \
\u2208 &isin\; \
\u2209 ¬in\; \
\u220b &ni\; \
\u220f &prod\; \
\u2211 &sum\; \
\u2212 &minus\; \
\u2217 &lowast\; \
\u221a &radic\; \
\u221d &prop\; \
\u221e &infin\; \
\u2220 &ang\; \
\u2227 &and\; \
\u2228 &or\; \
\u2229 &cap\; \
\u222a &cup\; \
\u222b &int\; \
\u2234 &there4\; \
\u223c &sim\; \
\u2245 &cong\; \
\u2248 &asymp\; \
\u2260 &ne\; \
\u2261 &equiv\; \
\u2264 &le\; \
\u2265 &ge\; \
\u2282 &sub\; \
\u2283 &sup\; \
\u2284 &nsub\; \
\u2286 &sube\; \
\u2287 &supe\; \
\u2295 &oplus\; \
\u2297 &otimes\; \
\u22a5 &perp\; \
\u22c5 &sdot\; \
\u2308 &lceil\; \
\u2309 &rceil\; \
\u230a &lfloor\; \
\u230b &rfloor\; \
\u2329 &lang\; \
\u232a &rang\; \
\u25ca &loz\; \
\u2660 &spades\; \
\u2663 &clubs\; \
\u2665 &hearts\; \
\u2666 &diams\; \
\u0022 "\; \
\u0026 &\; \
\u003c <\; \
\u003e >\; \
\u0152 &OElig\; \
\u0153 &oelig\; \
\u0160 &Scaron\; \
\u0161 &scaron\; \
\u0178 &Yuml\; \
\u02c6 &circ\; \
\u02dc &tilde\; \
\u2002 &ensp\; \
\u2003 &emsp\; \
\u2009 &thinsp\; \
\u200c &zwnj\; \
\u200d &zwj\; \
\u200e &lrm\; \
\u200f &rlm\; \
\u2013 &ndash\; \
\u2014 &mdash\; \
\u2018 &lsquo\; \
\u2019 &rsquo\; \
\u201a &sbquo\; \
\u201c &ldquo\; \
\u201d &rdquo\; \
\u201e &bdquo\; \
\u2020 &dagger\; \
\u2021 &Dagger\; \
\u2030 &permil\; \
\u2039 &lsaquo\; \
\u203a &rsaquo\; \
\u20ac &euro\; ]
set reversemap [lreverse $map]
proc insertentities {string} {
variable map
return [string map $map $string]
}
proc removeentities {string} {
variable reversemap
return [string map $reversemap $string]
}
}