Regex Tester MCP Server

Regex Tester

Run Python re-style regular expressions on arbitrary text entirely on-platform: compile with optional flags, see whether and where a pattern matches, inspect capturing and named groups, enumerate every occurrence with spans, and optionally apply a replacement across the full string.

Capabilities

• Single-match modes: Try a pattern at the beginning of the string, require the pattern to cover the entire string, or locate the first match anywhere in the text. The response states clearly whether there was a match and returns the matched substring, start and end indices, every capturing group (including empty or non-participating groups as null in the list), and named group values as a dictionary.
• Find every occurrence: Walk the whole input in order, returning up to a configurable number of match records while still reporting the true total count of matches in the input. Each record includes the matched text, span, groups, and named groups—useful for parsing, tokenizing, or validating structured text.
• Replacement preview: Optionally supply a replacement template (including backreferences such as \1) to receive the full string after substitution, alongside the match listing, without calling any external service.
• Flags: Enable case-insensitive matching, multiline ^/$ behavior, dot-all mode so . includes newlines, and verbose patterns with inline comments and ignored whitespace—using either long names or the usual single-letter aliases.

Gotchas

• Input size and time limits: Very large inputs and pathological patterns can be slow; oversized text is rejected and long-running evaluation times out to keep the hub responsive.
• Python regex dialect: Patterns follow the Python re module (similar to PCRE in many cases but not identical). Lookbehind and other features match Python’s rules.
• Replacement: When using substitution, the replacement string follows re.sub semantics (including group references and \\ escaping).