Java Regex Tester
Test a Java (java.util.regex) regular expression against sample text — find(), matches() or lookingAt() results with match positions, capture groups, replaceAll output and a Pattern.compile snippet.
Input
The pattern as the regex engine sees it — single backslashes, like \d+, not the escaped Java string "\\d+".
Matcher.replaceAll syntax: $1 or ${name} insert a group, \ escapes the next character.
Output
| # | Matched Text | start() | end() | Capture Groups |
|---|---|---|---|---|
| No data yet | ||||
More ways to use this tool
REST API
curl -X POST https://api.iotools.cloud/v1/tool/java-regex-tester \
-H "Authorization: Bearer YOUR_API_KEY" \
-H "Content-Type: application/json" \
-d '{
"pattern": "(?<user>\\w+)@(?<domain>[\\w.]+)",
"mode": "find",
"testString": "Contact: hello@example.com, support@iotools.clou…",
"replacement": "${domain}:${user}"
}'Swap in your own key from your account. The tool's fields are the body — no wrapper.
Ask an AI agent
Use the IOTools `java-regex-tester` tool (Java Regex Tester) on this input:
YOUR_INPUT_HEREPaste this at any agent connected to the IOTools MCP server, then add your input.
Embed widget
<iframe
src="https://iotools.cloud/embed/java-regex-tester/"
width="100%" height="520" frameborder="0" scrolling="no" loading="lazy"
title="Java Regex Tester — iotools.cloud"
sandbox="allow-scripts allow-forms allow-same-origin allow-downloads allow-popups allow-popups-to-escape-sandbox"
allow="clipboard-write"
style="width:100%;border:1px solid #e5e7eb;border-radius:12px;overflow:hidden"></iframe>
<script src="https://iotools.cloud/embed.js" async></script>Drop this into your own page — free, no key required, just a link back.
| Cost per API/MCP call | From 5 credits |
|---|---|
| Need more credits? | View pricing |
Also available with
Guides
The Java Regex Tester runs a java.util.regex pattern against your text and shows what Java's Matcher would report: every match, its start() and end() offsets, and each capture group. It follows Java's dialect, not JavaScript's, so features such as possessive quantifiers (a++), atomic groups ((?>…)), \Q…\E quoting, POSIX classes like \p{Alpha} and class intersection ([a-z&&[^aeiou]]) behave the way they do in your Java code.
How to use it
- Type the pattern the way the regex engine sees it:
\d+, not the Java string"\\d+". - Pick the matcher method:
find()lists every match,matches()checks that the whole text matches,lookingAt()checks that the text starts with a match. - Tick any
Patternflags you use:CASE_INSENSITIVE,MULTILINE,DOTALL,COMMENTSorLITERAL. - Paste your input text. Results update as you type.
Add a replacement such as ${user}@example.org or $2-$1 to preview replaceAll. A Pattern.compile(...) line with the pattern already escaped as a Java string literal is generated for you to paste into code.
Java details it reproduces
$also matches just before a final line terminator, and.,^and$use Java's line terminators (\n,\r\n,\r,\u0085,- Group names follow Java's rule: letters and digits only, starting with a letter.
- Offsets are UTF-16 indices, the same as Java strings.
Limits
Inline flags such as (?i) work at the very start of the pattern only; use the checkboxes for the rest. \G, \X and Unicode blocks (\p{InGreek}) aren't supported. Case-insensitive matching uses Unicode case folding, where Java defaults to ASCII only. Patterns with a quantified group inside a quantifier, like (a+)+, are refused because they can hang; (a++)+ is fine.
Everything runs in your browser; nothing is uploaded.
Related tools
For JavaScript patterns use the Regex Tester. To check a pattern for catastrophic backtracking use the Regex ReDoS Vulnerability Detector, and to escape strings for Java code use the Java & .NET String Escape tool.