regex builder explainer

    1

    Build, debug, and audit flavor-specific regular expressions with ReDoS protection and step-by-step breakdowns.

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    regex builder explainer

    Example session with this skill installed

    Build a regex for Python that matches a 24-hour time format (HH:MM). It must exclude 24:00. Explain how it works and give me a test table.

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    • Compiled the regex builder explainer

    I have generated a Python-compatible regular expression for 24-hour time validation. This pattern uses non-capturing groups to handle the hour logic and ensures strict bounds to exclude 24:00.

    24-Hour Time Pattern (Python re)

    
# Python extended verbose pattern
pattern = r"""
^                          # Start of string
(?:                        # Start hour group
  [01][0-9]                # 00-19

  |                        # OR

  2[0-3]                   # 20-23
)                          # End hour group
:                          # Separator
[0-5][0-9]                 # Minutes 00-59
$                          # End of string
"""

    Pattern Breakdown

    ComponentTypeDescription
    ^AnchorEnsures the match starts at the beginning of the string.
    (?:[01][0-9]|2[0-3])Non-capturing GroupMatches 00-19 or 20-23. Prevents 24+.
    :LiteralMatches the colon separator.
    [0-5][0-9]Character ClassMatches minutes from 00 to 59.
    $AnchorEnsures no trailing characters after the minutes.

    Test Matrix

    InputExpectedResultReason
    00:00MatchPassValid start of day.
    13:45MatchPassValid afternoon time.
    23:59MatchPassValid end of day.
    24:00No MatchPassExcluded per requirements (24 is out of range).
    12:60No MatchPassInvalid minutes.
    9:05No MatchPassFails due to lack of leading zero.

    Next steps

    1. Copy the raw pattern: ^(?:[01][0-9]|2[0-3]):[0-5][0-9]$ for use in re.match().
    2. If you need to support optional leading zeros for hours (e.g., "9:05"), I can update the hour group.
    3. Use the re.VERBOSE flag in Python to maintain the comments for future readability.

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    What you get

    Audit patterns for catastrophic backtracking and provide safe rewrites.Deconstruct complex legacy regex into readable documentation tables.Generate test matrices to verify edge cases for new validation patterns.Port regex patterns between languages while respecting flavor constraints.

    About this skill

    The problem

    Writing complex regular expressions is error-prone, and maintaining them is a nightmare. Developers often resort to copying unverified patterns from the web that lead to production bugs or catastrophic backtracking vulnerabilities.

    What it does

    • Generates commented, readable patterns in extended mode for PCRE, JS, Python, Go, Java, and Rust.
    • Breaks down existing cryptic patterns into piece-by-piece tables explaining literals, groups, and anchors.
    • Identifies flavor-specific incompatibilities like lookbehind support or Unicode class differences.
    • Performs ReDoS safety audits to detect and fix nested quantifiers that cause performance hangs.
    • Generates comprehensive test matrices covering both matching and non-matching edge cases.

    Frameworks & tools

    PCRE, JavaScript (V8), Python re, Go RE2, Java, and Rust regex crates.

    Why this beats prompting it yourself

    General-purpose LLMs often hallucinate regex flavor support, claiming lookbehinds work where they do not. This skill enforces flavor-specific validation, labels technical assumptions for verification, and actively audits for catastrophic backtracking risks that standard prompts miss.

    Use cases

    • Building a secure email validation pattern that avoids ReDoS attacks.
    • Translating a complex PCRE pattern from a legacy PHP app to a Go microservice.
    • Debugging why a specific user input fails a validation group in a JavaScript frontend.
    • Documenting a complex log-parsing pattern for a shared team library.

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