Nicer way to access match results? - regex

Question

My requirement is to transform some textual message ids. Input is
a.messageid=X0001E
b.messageid=Y0001E
The task is to turn that into
a.messageid=Z00001E
b.messageid=Z00002E
In other words: fetch the first part each line (like: a.), and append a slightly different id.
My current solution:
val matcherForIds = Regex("(.*)\\.messageid=(X|Y)\\d{4,6}E")
var idCounter = 5
fun transformIds(line: String): String {
val result = matcherForIds.matchEntire(line) ?: return line
return "${result.groupValues.get(1)}.messageid=Z%05dE".format(messageCounter++)
}
This works, but find the way how I get to first match "${result.groupValues.get(1)} to be not very elegant.
Is there a nicer to read/more concise way to access that first match?

Answer 1

You may get the result without a separate function:
val line = s.replace("""^(.*\.messageid=)[XY]\d{4,6}E$""".toRegex()) {
"${it.groupValues[1]}Z%05dE".format(messageCounter++)
}
However, as you need to format the messageCounter into the result, you cannot just use a string replacement pattern and you cannot get rid of ${it.groupValues[1]}.
Also, note:
You may get rid of double backslashes by means of the triple-quoted string literal
There is no need adding .messageid= to the replacement if you capture that part into Group 1 (see (.*\.messageid=))
There is no need capturing X or Y since you are not using them later, thus, (X|Y) can be replaced with a more efficient character class [XY].
The ^ and $ make sure the pattern should match the entire string, else, there will be no match and the string will be returned as is, without any modification.
See the Kotlin demo online.

Answer 2

Maybe not really what you are looking for, but maybe it is. What if you first ensure (filter) the lines of interest and just replace what needs to be replaced instead, e.g. use the following transformation function:
val matcherForIds = Regex("(.*)\\.messageid=(X|Y)\\d{4,6}E")
val idRegex = Regex("[XY]\\d{4,6}E")
var idCounter = 5
fun transformIds(line: String) = idRegex.replace(line) {
"Z%05dE".format(idCounter++)
}
with the following filter:
"a.messageid=X0001E\nb.messageid=Y0001E"
.lineSequence()
.filter(matcherForIds::matches)
.map(::transformIds)
.forEach(::println)
In case there are also other strings that are relevant which you want to keep then the following is also possible but not as nice as the solution at the end:
"a.messageid=X0001E\nnot interested line, but required in the output!\nb.messageid=Y0001E"
.lineSequence()
.map {
when {
matcherForIds.matches(it) -> transformIds(it)
else -> it
}
}
.forEach(::println)
Alternatively (now just copying Wiktors regex, as it already contains all we need (complete match from begin of line ^ upto end of line $, etc.)):
val matcherForIds = Regex("""^(.*\.messageid=)[XY]\d{4,6}E$""")
fun transformIds(line: String) = matcherForIds.replace(line) {
"${it.groupValues[1]}Z%05dE".format(idCounter++)
}
This way you ensure that lines that completely match the desired input are replaced and the others are kept but not replaced.