Zero-width space

U+200B is a character with no drawing and no advance width. Its entire job is to tell the line breaker that a wrap is allowed here, which is the one thing a long unbroken string does not otherwise offer. Put one between the words of a camel-case identifier, after the slashes of a URL, or inside a hash, and the string starts wrapping at the places you chose instead of running off the side of its column. Nothing appears at the break: no hyphen, no dash, no gap, because there is no glyph to draw.

That silence is the difference from a soft hyphen, which is the same kind of character with the opposite manners. A soft hyphen also marks a permitted break and is also invisible until it is used, but when the line does break there, a hyphen is drawn. That is right for prose, where a hyphen tells the reader a word was split, and wrong for anything a reader might copy or type back: a hyphen in the middle of ProductCatalogSyncScheduler reads as part of the name. Where a break must never happen at all, the mirror image is a non-breaking space, a character that occupies width and forbids the break its ordinary twin would allow.

The alternative to placing characters by hand is asking CSS to break anywhere: overflow-wrap: anywhere, or word-break with break-all. That needs no editing of content, and it also has no judgement, so it will split a name mid-syllable and hyphenate nothing. A zero-width space is the opposite trade: you say where the seams are, once, and every line thereafter breaks somewhere a reader would have chosen. For generated strings, inserting one after each separator is a one-line transform.

The cost is that the character is really there. It survives copy and paste, so a pasted identifier can carry invisible characters into a code editor, a search field, or a database, where it will not match the string it looks identical to. It counts toward a length limit, it can break an exact-match query, and it is a known trick for slipping past naive text filters. Use it in text you are rendering for reading, strip it from anything you are storing or comparing, and prefer the CSS route wherever the content is not yours to edit.