Regex Identifier

Identifiers (officially resource locations) are used all over Minecraft for identifying stuff like items, blocks, entities, loot tables and so on.
They consist of a namespace and a path, separated by a colon. Full documentation on identifiers can be found here.

Loot modifier predicates often use regex identifiers for matching these identifiers.
A regex identifier allows matching either a literal identifier or a regex pattern.

Instead of matching an exact piece of text, regex allows describing a pattern to match many possible variations.
For that, regex uses metacharacters. The most useful combination of which for matching identifiers is .*, which matches any character from zero to infinite times.
This is very useful for loot modifier predicates because you can build patterns like minecraft:.*_ingot for matching any ingot from minecraft.

Full documentation on supported regex syntax can be found here, and you can use a site like regex101 to write and test you regular expressions (just make sure to choose the Java 8 flavor)

A RegexIdentifier in json can either be an inlined string or an object with the regexPattern field.
Examples:

{

  "value1": "minecraft:diamond_sword",


  "value2": {
    "regexPattern": "minecraft:.*_sword"
  },


  "value3": {
    "regexPattern": "minecraft:.*"
  },


  "value4": {
    "regexPattern": "minecraft:oak_.*"
  }
}