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.
| 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:
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:
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:
✅ PCRE: Works
❌ .NET: “Unrecognized grouping construct”
Why it breaks: .NET only supports (?<name>...) and (?'name'...).
5. Ungreedy mode modifier ((?U))
PCRE supports:
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:
✅ 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:
✅ PCRE: Works
❌ .NET: “Unknown property”
Why it breaks: .NET’s regex engine is tied to older Unicode property tables.
