extract data from string

Hi [mention:50db1cd8fb304dd499245322b8cbac67:e9ed411860ed4f2ba0265705b8793d05],

Can you tell from where this string is coming?

Hi thiliniu0001,
you can utilize replace function to format existing string together with keyval("hello=alpha][goodbye=beta]", {"hello"}, "=", "]") to have this as result

Hi [mention:50db1cd8fb304dd499245322b8cbac67:e9ed411860ed4f2ba0265705b8793d05],

I am not sure, from where are you getting the input, and if the keys would always be same or different. 
Considering the input, keys and format same,
You can try the below code 

a!localVariables(
  /*The string input*/
  local!string: "[place_id:519fcdaacfd556c0bf591a4bfd61f0c04940f00101f9014600010000000000c002089203064c6f6e646f6e,address_line1:London,city:London,address_line2:ENG, United Kingdom,country:United Kingdom,state:England,county:Greater London,formatted:London, ENG, United Kingdom]",
  /*The Keys*/
  local!keys: {
    "place_id",
    "address_line1",
    "city",
    "address_line2",
    "country",
    "state",
    "county",
    "formatted"
  },
  /*Removing the [ , ]*/
  local!strippedString: stripwith(local!string, "[]"),
  /*Storing the patterns in a variable*/
  local!patterns: {
    "place_id:[^,]+",
    "address_line1:[^,]+",
    "city:[^,]+",
    "address_line2:[^,]+,[^,]+,",
    "country:[^,]+",
    "state:[^,]+",
    "county:[^,]+",
    "formatted:[^,]+,[^,]+,[^,]+"
  },
  local!subStrings: a!flatten(
    a!forEach(
      items: local!patterns,
      expression: regexsearch(
        pattern: fv!item,
        searchString: local!strippedString,
        regexFlags: "g"
      ).match,

    )
  ),
  createdictionary(
    local!keys,
    a!flatten(
      a!forEach(
        items: local!keys,
        expression: keyval(
          local!subStrings[fv!index],
          fv!item,
          ":",
          ""
        )
      )
    )
  )
)