RegexSmith

Why RegexSmith

Features

• The hover card over a pattern in C#: the pattern with its groups colored, a line per piece in plain words, colored group tags and a status — ✓ safe, ⚠ may hang or ✕ not valid.
• Group underlines: a thin line in the group's color under each capture group right in the code; nested groups sit lower.
• Samples made from the pattern — two strings that match and one that does not — checked by the real .NET engine.
• RX001: a squiggle under repetitions that can take exponential time, with fixes in Ctrl+. and a before/after preview.
• The RegexSmith Tester, docked next to Find Results: your samples with ✓/✗ expectations, matches and groups highlighted, a groups table and a Replace tab.
• Apply to code: the edited pattern goes back into the same kind of string literal, with its options, as one Undo.
• Copy as unit test for xUnit, NUnit or MSTest — your samples become test cases.

The hover card

Hover the string of a pattern, and the usual tooltip of Visual Studio gets a RegexSmith card: what the pattern means, what it matches and whether it is safe.

Where it works

In C# files, over the pattern of:

• new Regex("…"), with or without RegexOptions;
• the static methods Regex.IsMatch, Match, Matches, Replace, Split and Count;
• [GeneratedRegex("…")];
• [RegularExpression("…")] — the card then says Validation: the whole value must match;
• a const string in the same file that one of these uses.

Regular, verbatim (@"…") and raw ("""…""") strings are read the way the compiler reads them, so "\\d" and @"\d" are the same pattern.

What the card shows

The status

"Safe" means only that: .NET accepts the pattern and it cannot hang. It does not mean that it matches what you intend — that is what the samples and the tester are for.

Group underlines

Every capture group of a pattern gets a thin line in its color right in the code — the same color as in the card. A long pattern with five groups can be read at a glance: which part is the year, which is the id.

A group inside another sits a little lower, so both lines stay visible. The colors go in order: salmon, green, blue, violet, amber, teal — and again.

The underlines can be turned off in the settings: Underline capture groups.

Patterns that can hang

Some patterns take exponential time on a long string that almost matches — the classic way a regex freezes a web request. RegexSmith finds them by their shape and puts a warning squiggle under the repetition; hover it for the reason.

What is underlined (RX001)

• Nested repetition: (a+)+, ([a-z0-9]+\.?)+ — one run of the same characters can be split between the inner and the outer repetition in very many ways.
• Repetition of alternatives that match the same characters: (\w|\d)+ — a digit can be taken by either branch, so every digit doubles the work.

Not inside atomic groups or lookarounds, and never with RegexOptions.NonBacktracking — there it cannot happen.

Fixes in Ctrl+.

Put the caret on the squiggle and press Ctrl+. — each fix shows a preview: the pattern before (struck through) and after.

RX001 can be turned off in the settings (Hints in the editor) or with Don't show this kind of hint in its tooltip; the Ctrl+. fixes stay.

The tester

The RegexSmith Tester opens docked next to Find Results. The pattern comes from the code and stays linked to it: try it on your own samples, change it, and write it back.

Open it

• ▶ Test pattern… in the hover card;
• Ctrl+. on a pattern › Test this pattern in RegexSmith;
• right-click in a C# file › Test Regex Pattern;
• View › Other Windows › RegexSmith Tester — not linked to code: a place to try a pattern.

The pattern and its options

At the top, the file and line of the pattern — click to go there. Below, the pattern box, the status line (✓ safe, ⚠ may hang or the engine's error, then N of M as expected · ms) and the option chips: IgnoreCase, Multiline, Singleline, IgnorePatternWhitespace, ExplicitCapture, RightToLeft, CultureInvariant. Every change runs again at once.

Samples and expectations

The first time, the samples of the card are there: two that should match and one that should not. Type a new one in Add a sample and press Enter…; click a sample to edit it, ✕ removes it. The mark before each sample says what it should do — click it to change:

A sample that does the opposite turns red — ✗ matches or ✗ no match — so the samples become a tiny spec of the pattern. Matches are tinted, groups underlined in their colors, spaces shown as dots. A run that takes more than 1 second stops with ⏱ too slow.

Groups of the selected sample lists every match of the selected sample: the whole match, then each group with its value and position (not captured when it took no part).

Replace

The Replace tab adds a replacement box — $1, ${name}, $$ for a $ — and shows the result for each sample, as Regex.Replace would.

Copy, revert, apply

Copy as unit test puts a test with your samples on the clipboard, for xUnit, NUnit or MSTest — the spec moves into the test project.

Revert goes back to the pattern and options from the code; the samples stay.

Apply to code writes the pattern back as the same kind of literal it was (regular, verbatim or raw) together with the options — one Undo takes it back. If that code changed after the tester opened it, nothing is written: open the tester from the pattern again.

Samples are remembered per pattern and per solution in %LocalAppData%\RegexSmith\samples, so the next time they are already there. Nothing goes into the repository.

Settings

Tools › Options › SmithWorks › RegexSmith. Every option has a short description of what it does.

Option	Default
Explain patterns on hover	On
Underline capture groups	On
RX001 Catastrophic backtracking	On
Ask for a rating on the Marketplace	On

At the bottom: Reset to defaults, Documentation, Rate on Marketplace and Send feedback.

Settings are stored in %LocalAppData%\RegexSmith.

FAQ

Changelog

1.0.1

• Hints in the editor have their own list on the options page, and a tooltip link turns a kind of hint off where you see it.

1.0.0

• First release: the hover card with a plain-language breakdown, colored groups, samples checked by the .NET engine and a status; group underlines in the code; RX001 for patterns that can hang, with atomic and NonBacktracking fixes in Ctrl+.; the RegexSmith Tester with expectations, groups, Replace, Copy as unit test and Apply to code; a one-time request for a rating.