Unicode regular expressions are essential for working with text in multiple languages and character sets. As the world becomes more interconnected, supporting Unicode is increasingly important for ensuring that software can handle diverse text inputs.

What is Unicode?

Unicode is a standardized character set that encompasses characters and glyphs from all human languages, both living and dead. It aims to provide a consistent way to represent characters from different languages, eliminating the need for language-specific character sets.

Challenges with Unicode in Regular Expressions

Working with Unicode introduces unique challenges:

1. Characters, Code Points, and Graphemes:

   • A single character (grapheme) may be represented by multiple code points. For example, the letter "à" can be represented as:
     • A single code point: U+00E0
     • Two code points: U+0061 ("a") + U+0300 (grave accent)
   • Regular expressions that treat code points as characters may fail to match graphemes correctly.

2. Combining Marks:

   • Combining marks are code points that modify the preceding character. For example, U+0300 (grave accent) is a combining mark that can be applied to many base characters.

Matching Unicode Graphemes

To match a single Unicode grapheme (character), use:

• Perl, RegexBuddy, PowerGREP: \X
• Java, .NET: \P{M}\p{M}*

Example:

\X matches a grapheme
\P{M}\p{M}* matches a base character followed by zero or more combining marks

Matching Specific Code Points

To match a specific Unicode code point, use:

• JavaScript, .NET, Java: \uFFFF (FFFF is the hexadecimal code point)
• Perl, PCRE: \x{FFFF}

Unicode Character Properties

Unicode defines properties that categorize characters based on their type. You can match characters belonging to specific categories using:

• Positive Match: \p{Property}
• Negative Match: \P{Property}

Common Properties:

\p{L} - Letter
\p{Lu} - Uppercase Letter
\p{Ll} - Lowercase Letter
\p{N} - Number
\p{P} - Punctuation
\p{S} - Symbol
\p{Z} - Separator
\p{C} - Other (Control Characters)

Unicode Scripts and Blocks

Unicode groups characters into scripts and blocks:

• Scripts: Collections of characters used by a particular language or writing system.
• Blocks: Contiguous ranges of code points.

Example Scripts:

\p{Latin}
\p{Greek}
\p{Cyrillic}

Example Blocks:

\p{InBasic_Latin}
\p{InGreek_and_Coptic}
\p{InCyrillic}

Best Practices for Unicode Regex

1. Use \X to match graphemes when supported.
2. Be aware of different ways to encode characters.
3. Normalize input to avoid mismatches due to different encodings.
4. Use Unicode properties to match character categories.
5. Use scripts and blocks to match specific writing systems.