Advanced Regex Replace Tester Calculator

Regex Replace Tester Form

Use the fields below to validate a pattern, apply replacements, inspect groups, and export the resulting audit details.

Regex Pattern

Enter the body only. Delimiters are selected separately.

Replacement String

Use backreferences like $1, $2, and $0.

Subject Text

Delimiter

Choose a delimiter that conflicts least with your pattern.

Modifiers

Common flags: i, m, s, x, u, A, D, U, J.

Replacement Limit

Use -1 to replace every match.

Reset Fields

Example Data Table

Use Case	Pattern	Replacement	Modifiers	Sample Outcome
Email masking	`(\w+)@(\w+\.\w+)`	`[hidden email: $1 at $2]`	`i`	Obscures addresses while preserving readable parts.
Ticket renaming	`BUG-(\d+)`	`ISSUE-$1`		Standardizes issue prefixes across text exports.
Whitespace cleanup	`\s+`		`m`	Compresses repeated spacing into single spaces.
HTML heading swap	`<h1>(.*?)</h1>`	`<h2>$1</h2>`	`is`	Rewrites heading levels while preserving content.

Formula Used

The tester follows this replacement expression:

Output = preg_replace(compiledPattern, replacement, subject, limit, replacementsMade)

In practical terms, the calculator compiles a complete regex from your delimiter, pattern body, and modifiers. It then scans the subject text, replaces matched segments up to the supplied limit, counts replacements, and compares output length against original length.

Supporting diagnostics use preg_match_all to list matches, offsets, and captured groups. That gives developers a reliable preview before moving the same logic into production code, tests, data cleanup scripts, or migration utilities.

How to Use This Calculator

Paste or type the source text into the subject field.
Enter the regex body without surrounding delimiters.
Choose a delimiter that does not collide with the pattern.
Add any needed modifiers such as i, m, s, or u.
Write the replacement string and include backreferences when needed.
Set the replacement limit, or use -1 for all matches.
Press Submit Tester to render the result above the form.
Review replacements, captured groups, match offsets, and length changes.
Use the CSV or PDF buttons to save the generated output summary.

Frequently Asked Questions

1. What does this tester actually validate?

It validates the regex syntax, applies the replacement, counts matches, shows captured groups, and compares original versus transformed text. This helps you catch faulty patterns before they reach application code.

2. Can I use backreferences in replacements?

Yes. Use values such as $1, $2, or $0 in the replacement field. The tester keeps those references intact and shows whether they produce the expected output.

3. Why are delimiters chosen separately?

Separating delimiters makes pattern entry cleaner and reduces typing mistakes. It also lets you switch delimiters when your pattern already contains slashes, hashes, or similar characters.

4. What does a replacement limit of -1 mean?

A limit of -1 tells the engine to replace every match it finds. Positive values restrict how many replacements occur, which is useful for incremental tests or partial edits.

5. Does the tester show every match location?

Yes. The result table lists each full match with its byte offset and any captured groups. That makes debugging easier when repeated patterns appear in long strings.

6. When should I enable modifiers like i or s?

Use i for case-insensitive matching, m for multiline anchors, s when dots should span new lines, and u for Unicode-safe processing.

7. What happens if my pattern is invalid?

The page shows a validation error above the tester. It also surfaces the regex engine message when available, helping you correct broken groups, escapes, quantifiers, or unsupported constructs.

8. Are the CSV and PDF downloads useful for teams?

Yes. CSV is convenient for result logs, QA notes, or audit trails. PDF works well for sharing a clean snapshot of inputs, replacements, counts, and final output during reviews.