.NET regular expressions that break Perl Compatible Regular Expressions (PCRE) standard

Examples of patterns that work in PCRE but break in .NET

The Perl Compatible Regular Expressions (PCRE) library is a set of functions that implement regular expression pattern matching using the same syntax and semantics as Perl 5. PCRE has its own native API, as well as a set of wrapper functions that correspond to the POSIX regular expression API.
The current version of PCRE2 is version 10.47 and released Oct 21, 2025.

Good news a clever programmer created a dotnet wrapper for the PCRE2 library used in Perl supporting 10.47! https://github.com/ltrzesniewski/pcre-net/, 1st version v0.1.0 requires .NET 4.5 Framework.

Pattern Type	PCRE	.NET	Why It Breaks
Possessive quantifiers	✅	❌	Not implemented
Variable‑length lookbehind	✅	❌	.NET requires fixed length
Subroutine calls / recursion	✅	❌	Feature not supported
Python‑style named groups	✅	❌	.NET uses different syntax
Ungreedy mode ((?U))	✅	❌	No equivalent in .NET
Recursive named groups	✅	❌	Not implemented
Modern Unicode properties	✅	❌	.NET uses older Unicode
\R newline escape	✅	⚠️	Only supported in newer .NET
Lookbehind with alternation	✅	❌	Variable length

1. Possessive quantifiers (++, *+, ?+, {m,n}+)

PCRE supports:

^\w++$

Meaning: match a word string with no backtracking allowed.

✅ PCRE: Works ❌ .NET: Throws “quantifier following nothing” or treats ++ as literal + depending on context.

Why it breaks: .NET simply does not implement possessive quantifiers.

2. Variable‑length lookbehind

PCRE allows:

(?<=\w+)\d+

Meaning: match digits preceded by one or more word characters.

✅ PCRE: Works ❌ .NET: “Lookbehind assertion is not fixed length”

Why it breaks: .NET requires lookbehind to have a fixed, compile‑time length.

3. Subroutine calls / recursion

PCRE supports recursive patterns:

^(\w+|(\((?1)\)))+$

Used for matching nested parentheses.

✅ PCRE: Works ❌ .NET: “Unrecognized grouping construct”

Why it breaks: .NET does not support (?1), (?&name), or recursive patterns at all.

4. Python‑style named groups ((?P<name>...))

PCRE supports:

(?P<word>\w+)-(?P=word)

✅ PCRE: Works ❌ .NET: “Unrecognized grouping construct”

Why it breaks: .NET only supports (?<name>...) and (?'name'...).

5. Ungreedy mode modifier ((?U))

PCRE supports:

(?U)^.+$

Meaning: make all quantifiers lazy by default.

✅ PCRE: Works ❌ .NET: “Unrecognized inline modifier”

Why it breaks: .NET has no equivalent to PCRE’s global ungreedy mode.

6. Recursive named groups ((?&name))

PCRE:

(?<paren>\((?:[^()]++|(?&paren))*\))

✅ PCRE: Works ❌ .NET: Fails immediately

Why it breaks: .NET has no syntax for recursive named groups.

7. Newline escape \R in older .NET versions

PCRE:

^\R$

✅ PCRE: Works ❌ .NET (pre‑.NET 7): “Unrecognized escape sequence”

Why it breaks: .NET added \R only recently.

8. Lookbehind with alternation of different lengths

(?<=abc|z)\d+

✅ PCRE: Works ❌ .NET: “Lookbehind assertion is not fixed length”

Why it breaks: Alternation creates variable‑length lookbehind.

9. Unicode property escapes beyond Unicode 4.0.1

PCRE supports modern Unicode categories:

\p{Script=Greek}

✅ PCRE: Works ❌ .NET: “Unknown property”

Why it breaks: .NET’s regex engine is tied to older Unicode property tables.