Keyboard Maestro uses ICU Regular Expressions (aka RegEx or RegExp) which is very similar to PCRE (Perl Compatible Regular Expressions), and you can read their documentation by choosing ICU Regular Expression Reference from the Help menu in Keyboard Maestro.
In Keyboard Maestro actions, if it uses the term “matches” is means a regular expression.
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Info to consider for inclusion here (in no order, numbered for easy reference)
Example #1: Remove all line numbers from a string with multiple lines
Example #2: Extract Capture Group for Multiple Matches to Multiple Lines in a Variable
Sometimes you may need to extract a RegEx Capture Group after a match is made, and do this for multiple lines (matches) in the entire source string. To achieve this, you need to:
Both Actions can use the same RegEx pattern.
For a complete macro using this method, see:
MACRO: Get List of RegEx Capture Group of Multiple Matches
Keywords: Regular Expression, RegEx, RegExp, Find, Replace, Match