On my list of things to try out when I need to use regular expressions again:

Test your regex by visualizing it with a live editor. JavaScript, Python, and PCRE.

Source: Debuggex: Online visual regex tester. JavaScript, Python, and PCRE.

Via: [WayBack] Sergiu Toarca answering [WayBack] Regex lookahead with multiple negative conditions – Stack Overflow

Note: in January 2018, Debuggex was down for a few days (Thanks Uwe for bringing that up and giving the below alternative [WayBack]).

If you are on Windows and are OK with a small cost, try this instead: [WayBack] RegexBuddy: Learn, Create, Understand, Test, Use and Save Regular Expression

–jeroen

Since I will never be good at regex, this are a few search/replace patterns I used in Atom.io to convert a plain text document space indented like this:

Version 1.23.4.15
First level indented text
Is also a first level indented text
Too a first level indented text
- First level indented text too
  Second level indented text
  Is also a second level indented text
  Too a second level indented text
  - Second level indented text too
    - A third level indented text
    Third level indented text
    Is also a third level indented text
    Too a third level indented text
98.4.32.1

to a Markdown indented one like this:

Version 1.23.4.15
- First level indented text
- Is also a first level indented text
- Too a first level indented text
- First level indented text too
 - Second level indented text
 - Is also a second level indented text
 - Too a second level indented text
 - Second level indented text too
 - A third level indented text
 - Third level indented text
 - Is also a third level indented text
 - Too a third level indented text
Version 98.4.32.1

Prepend Version when needed: https://regex101.com/r/CUhUbr/1

• Search:
  • ^(\d+\.\d+\.\d+\.\d+)
• Replace:
  • Version $1

Add markdown first level indentations to lines that don’t have a markdown first level indentation yet, nor start with Version nor start with a space: https://regex101.com/r/CUhUbr/2

• Search:
  • ^(?!(Version )|(- )|( )|(\d+\.\d+\.\d+\.\d+))(.*)
• Replace:
  • - $5

Add markdown second level indentations to lines that don’t have a markdown second level indentation yet but do have a regular second level indentation: https://regex101.com/r/CUhUbr/3

• Search:
  • ^  (?!(- )|( ))(.*)
• Replace:
  •   - $3

Add markdown third level indentations to lines that don’t have a markdown third level indentation yet but do have a regular third level indentation: https://regex101.com/r/CUhUbr/4

• Search:
  • ^    (?!(- )|( ))(.*)
• Replace:
  •     - $3

––jeroen

https://regex101.com/r/CUhUbr/1

https://regex101.com/r/CUhUbr/2

https://regex101.com/r/CUhUbr/3

https://regex101.com/r/CUhUbr/4