Understanding Posix Standards For Basic, Extended, And Perl-Compatible Regular Expressions

The POSIX (Portable Operating System Interface) standards specify several flavors of regular expressions used in programming languages and text processing. This article will explore the history, core syntax, and key features of basic POSIX regular expressions, extended POSIX regular expressions, and Perl-compatible regular expressions (PCRE).

History of POSIX and regex standards

The Institute of Electrical and Electronics Engineers (IEEE) adopted the POSIX standards in 1988 to define a portable operating system interface across UNIX and other operating systems. These standards included specifications for basic and extended regular expression matching capabilities.

Perl-compatible regular expressions (PCRE) emerged in 1997 when the original creator of the Perl programming language developed a C library for implementing Perl’s powerful regex syntax. PCRE became widely adopted across programming languages and tools.

Understanding the POSIX regex standards allows developers to write portable regular expressions. Familiarity with PCRE enables the use of advanced pattern matching capabilities.

Basic POSIX regular expressions

The basic POSIX regular expressions provide core pattern matching functionality with a limited set of metacharacters. Key capabilities include anchors, character classes, quantifiers, and grouping.

Anchors

Anchors do not match any character, but instead match positions in the string:

• ^ – Matches the start of the string
• $ – Matches the end of the string

This allows matching if a pattern appears at the beginning or end of the search text.

Character classes and ranges

Character classes distinguish types of characters:

• [0-9] – Matches any digit
• [a-z] – Matches any lowercase letter
• [A-Z] – Matches any uppercase letter

Ranges match any character alphabetically between the two specified characters.

Common metacharacters

Special metacharacters enable positional matching:

• . – Matches any single character
• ^ – Matches start of the string
• $ – Matches end of the string

These complement character classes for flexible partial matching.

Grouping and alternation

Grouping constructs allow combining expressions:

• ( ) – Group a series of expressions
• (a|b) – Match either a or b

Grouping and alternation provides subexpression reuse and options for matching.

Extended POSIX regular expressions

Extended POSIX regular expressions build upon the basics with metacharacters for context sensitivity, greediness control, and enhanced grouping functionality.

Additional metacharacters

New metacharacters available:

• ? – Match preceding item 0 or 1 times
• + – Match preceding item 1 or more times
• {n} – Match preceding item n times
• {n,m} – Match preceding item n to m times

These quantitative metacharacters offer more control over qualifying matches.

Context sensitivity

Metacharacters such as \b allow positional matching based on context:

• \b – Empty string at either edge of a word
• \B – Empty string not at an edge of a word

This enables expressions to distinguish word boundaries.

Greediness and laziness

The * and + quantifiers are greedy, matching as much as possible. The non-greedy versions *? and +? match as little as possible.

Tuning greediness prevents excessive matching.

Perl-compatible regular expressions (PCRE)

Perl-compatible regular expressions build upon the extended POSIX syntax with additional capabilities.

Lookaround assertions

Lookaround assertions match patterns before or after the main expression without including them in the match:

• (?= ) – Positive lookahead (asserts that pattern exists ahead)
• (?! ) – Negative lookahead (asserts that pattern does not exist ahead)

This allows matching dependent on assertions about adjoining text.

Named capture groups

Capture groups can be named for easier handling of matches:

• (?<name> ) – Named capturing group

Instead of match position numbers, groups can have semantic names.

Recursive patterns

Subexpressions can recurse, with infinite recursion prevented:

• (?R) – Recursive subexpression call
• (?1) – Recursive call to first subexpression

This allows matching of nested constructs.

Regex engines and standards conformance

Most modern regex engines support at least extended POSIX syntax. Some additional features found:

• Unicode character matching
• Vertical whitespace matching
• Leftmost greedy/lazy matching

Fine variations exist in metacharacter handling between implementations.

Writing standards-compliant regexes

Keep these guidelines in mind when aiming for portable regular expressions:

Literal meaning

Some metacharacters lose literal meaning:

• . ( ) | ^ $ * + ? [ ] { } \

Use escapes like \. or character classes to match literally.

Quantifiers

Quantifier application differs among engines. Always qualify with explicit greediness.

Character encoding

Use Unicode character classes like \p{L} if encoding permits. Otherwise stick to ASCII [a-zA-Z].

Example regex codes

Some applied regular expression examples:

Basic regex for email validation

^[a-zA-Z0-9_.+-]+@[a-zA-Z0-9-]+\.[a-zA-Z0-9-.]+$

• Username
• @ symbol
• Domain name
• . period
• Top-level domain

Extended regex for HTML tag matching

<(\w+)(?:(?<attrs>(\w+="[^\"]*")*?)|(?<selfclosed>\/)?)>

• Opening angle bracket
• Tag name
• Attributes group
• Self-closing forward slash
• Closing angle bracket

PCRE regex for parsing log files

  
^\[(?<timestamp>.*\d\d:\d\d:\d\d).*\]\s\[(?<level>.*)\]\s(?<message>.*)

• Opening log timestamp
• Named capturing group for timestamp
• Closing log timestamp
• Log level named capture
• Log message named capture