Regular Expressions

Regular expressions are a powerful tool for pattern matching and text manipulation. They are used in many programming languages, text editors, and command-line tools.

• testing regex with specialized tools
  • rexi
    • Terminal UI for Regex Testing. An interactive CLI for testing regex from terminal.
    • ^more

      # installation
      pip3 install rexi
      # usage
      cat /etc/hosts | rexi
      

  • sig
    • Interactive grep (for streaming)
    • ^more

      sig is an interactive grep tool designed for streaming data. It offers real-time search updates as data streams in, making it easy to navigate large amounts of dynamic data.

      brew install ynqa/tap/sigrs
      

      # press `<ctrl-f>` to enter Archived mode
      curl https://raw.githubusercontent.com/ynqa/sig/0db6dda7ddc6e9b9197e0d249e3f31e73b3d0d50/README.md |& sig
      # press `<ctrl-f>` to enter Archived mode
      sig --cmd "curl https://raw.githubusercontent.com/ynqa/sig/0db6dda7ddc6e9b9197e0d249e3f31e73b3d0d50/README.md"
      # archived mode
      sig -a --cmd "ls -la"
      ls -la |& sig -a
      

      This tool also features keyboard shortcuts, an archived mode for dealing with backwards searching issues related to piping processes. In archived mode, sig saves the latest N entries and allows searching through them based on given key inputs. This mode also enables users to search through static data, such as files.

      sig works best when used in environments where data is streamed, such as the output of kubernetes logs or the output of websocket data.

  • RegExr
    • RegExr is an online tool to learn, build, & test Regular Expressions (RegEx / RegExp).
• programming languages
  • JavaScript
    • MDN
    • internal link
      • browser JS
• Concepts ¹
  • Back References: Matching the same text captured earlier in the pattern.
    • Backreferences allow you to match the same text that was previously matched by a capturing group. In a regex pattern, you can refer to a capturing group by using a backslash followed by its group number. Group numbers are assigned based on the order of the opening parentheses in the pattern, starting from 1.
  • Assertions: Checking conditions before or after a match without including them in the match.
    • There are two main types of assertions: lookaheads and lookbehinds.
    • Assertions are patterns that check for conditions before or after a match. Once a regex matches, assertions verify if it’s preceded or followed by a specific pattern.
    • While they are considered during the search, they aren’t included in the final match. There are two main types of assertions: lookaheads and lookbehinds.
  • Anchors: Using ^ and $ to match the start and end of lines, respectively.
  • Quantifiers: Specifying how many times a character or group should appear.
  • Non-Capturing Groups: Grouping without capturing for performance optimization.
    • By default, any group you define in a regular expression captures the matched expression for use in subsequent parts of the pattern. If you don’t want the group to capture, simply prefix the pattern inside the parentheses with ?:
      • This allows for better performance.
      • e.g. /^(?: hello) world $/

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1. Learning These Things Made Me A RegEx Expert ↩