Unicode Line Breaking Algorithm

Breaking Text Across Lines

When a paragraph of text is too wide for its container, a rendering engine must decide where to insert line breaks. This sounds straightforward but involves subtle rules that vary by script, language, and context. The Unicode Line Breaking Algorithm (UAX #14) provides a systematic framework for finding valid break opportunities.

The algorithm does not tell renderers where to break — that depends on line width and font metrics. Instead, it classifies each position between characters as a mandatory break, a break opportunity (the renderer may break here), or no break (breaking here is not allowed).

Line Break Classes

Each Unicode character is assigned a line break class. There are over 40 classes; the most important:

The key insight for international text: CJK ideographic characters (Chinese, Japanese, Korean) have a break opportunity after almost every character because those scripts historically had no spaces. Latin letters have no break opportunities between them — only at space or hyphen positions.

Connection to CSS

The UAX #14 algorithm is the foundation for CSS line-breaking properties:

/* Allow break at any character (overrides UAX #14 for CJK) */
word-break: break-all;

/* Break long words that would overflow */
overflow-wrap: break-word;

/* Strict CJK line breaking (no breaks before small kana) */
line-break: strict;

/* Loose CJK line breaking (more break opportunities) */
line-break: loose;

/* Prevent breaking across lines entirely */
white-space: nowrap;

Python and Line Breaking

Python's textwrap module uses a simplified version of line breaking for ASCII-centric text. For full UAX #14 compliance, the uharfbuzz or PyICU libraries provide proper implementations:

import textwrap

# Basic wrapping (works for Latin text)
text = "The quick brown fox jumps over the lazy dog."
print(textwrap.fill(text, width=30))

# For CJK text, textwrap.fill does NOT insert breaks
# between ideographs — use a proper UAX #14 implementation
# (uharfbuzz, or browser rendering)

Quick Facts