Matching a pattern at the beginning of a string via the ^ regex token is pretty straight forward.

Example:

const regex = /^theThing/

const string = "theThing lol hehehe"

string.match(reg)
// > ["theThing", index: 0, input: "theThing lol hehehe", groups: undefined]

However, what if we want to match a given string but only if it isn't at the start of the string?

The regex below allows matching pattern that is not at the start of a string or line via (?<!^) regex tokens.

Example:

const regex = /(?<!^)> function /

const string = "> function ${merge('x', > function merge('y', 'z'))}"

string.match(regex)
// ["> function ", index: 24, input: "> function ${merge('x', > function merge('y', 'z'))}", groups: undefined]

const stringTwo = "> function ${merge('x', 'x')}"
stringTwo.match(regex)
// null

The use case here would be trying to find a nested pattern in a string via regex.

This was handy for implementing recursive function config params in my configorama library.