Examples of patterns that work in PCRE but break in .NET
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
| 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:
✅ 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.
