library(stringr)
rx <- "(\\d{4})(\\d{2})(\\d{2})_(\\d{2})(\\d{2})(\\d{2})(?:_(\\d+))?"
Filenames = c("blabla_bli_20140524_002532_000.wav", "20201025_231205.wav", "ble_20190612_220013_012.wav", "X-20150312_190225_Blablu.wav", "0000125.wav")
m <- str_match(Filenames, rx)
result <- ifelse(is.na(m[,8]),
str_c(m[,2], "-", m[,3], "-", m[,4], " ", m[,5], ":", m[,6], ":", m[,7], ".000"),
str_c(m[,2], "-", m[,3], "-", m[,4], " ", m[,5], ":", m[,6], ":", m[,7], ".", m[,8]))
result
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