Regex Tool: Pattern Editor Help & FAQ

Build, test, and debug regular expressions on iPhone and iPad with live match highlighting, named groups, and a built-in cheat sheet.

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Frequently asked questions

Which regex flavor does the app use?

Regex Tool uses ICU regex (the same engine as NSRegularExpression on iOS, similar to PCRE2). Most patterns you find online for JavaScript, Java, or Python work directly. The cheat sheet inside the app calls out the small differences (like \K not being supported).

Does it support named capture groups and backreferences?

Yes. You can write (?\d{4}) to name a group and \k or $1 to reference it in replacements. The match panel shows each named group separately so you do not have to count parentheses.

Can I do replace, not just match?

Yes. The Replace tab lets you supply a replacement template (with $1, $2, ${name} placeholders) and shows the live result. You can copy the entire transformed text in one tap.

Why is my pattern not matching what I expect?

The two most common reasons: the dot (.) by default does not match newlines (use the s flag), and \d in ICU does not include all Unicode digits unless you opt in. The app shows the active flags at all times so you can verify them.

Does it work offline?

Yes. The matching engine is built in. No network connection is needed and your test text never leaves the device.

Can I save patterns I use often?

Yes. Tap the bookmark icon on any pattern to save it to your library. Saved patterns sync across iPhone and iPad through iCloud if you are signed in.

Which flags are supported?

Case-insensitive (i), multiline (m), dotall (s), Unicode (u), and extended/comments (x). Flags are toggleable from the toolbar so you do not need to write inline modifiers like (?i).

Regex Tool: Pattern Editor

Get Regex Tool: Pattern Editor

Regular-expression playground with live highlights, named groups, and replace mode.

How-to guides

How to test a regex pattern on iPhone

Build and verify a pattern in seconds with live highlighting.

  1. Open Regex Tool and tap the New Pattern button.
  2. Type your pattern in the top field (for example, \b\w+@\w+\.\w+\b).
  3. Paste sample text into the test area below.
  4. Watch each match highlight as you type. Capture groups are listed under the match.
  5. Toggle the i, m, or s flags from the toolbar if you need different matching behavior.
Try this in Regex Tool: Pattern Editor →

How to extract all email addresses from a block of text

A common real-world task that takes one pattern and one tap.

  1. Paste the block of text into the test area.
  2. Use the pattern: [a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}
  3. Make sure the global flag is on so all matches are listed (this is the default).
  4. Open the Matches panel and tap Copy All to put every match on the clipboard, one per line.
  5. Paste into Notes, Mail, or any other app.

For very strict email validation use a fuller pattern, but the one above catches almost everything you encounter in the wild.

Try this in Regex Tool: Pattern Editor →

Related guides

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