如何提取字符串第二个下划线后的数字？R语言stringr正则问题

Hey there! Let's figure out why your regex is returning NA and fix it up.

Your original regex (^_){2}(\\d+\\.*\\d*) is trying to match two underscores at the very start of the string (^ anchors to the start), but your input string starts with 1/2/3—that's why it can't find a match and returns NA.

Fix 1: Use a Positive Lookbehind #

This approach uses a lookbehind to "skip" everything up to the second underscore, then extracts the digits that follow:

library(stringr)
input <- "1/2/3_500k/855kk_1400k/AVBB"
# Extract digits after the second underscore
result <- str_extract(input, "(?<=^.*?_.*?_)(\\d+\\.*\\d*)")
print(result)
# Output: [1] "1400"

Breakdown:

• (?<=^.*?_.*?_): This positive lookbehind checks that the text before our target digits follows the pattern: start of string → any characters (non-greedily) → underscore → any characters → underscore. This effectively skips everything up to the second underscore.
• (\\d+\\.*\\d*): Matches integers or decimal numbers (so it works if you ever have something like 1400.5 instead of 1400).

Fix 2: Use str_match to Capture the Target Group #

If you prefer a more explicit approach, str_match lets you capture specific parts of the match using groups:

library(stringr)
input <- "1/2/3_500k/855kk_1400k/AVBB"
# Match up to the second underscore, then capture the following digits
match_matrix <- str_match(input, "^.*?_.*?_([0-9.]+)")
# Grab the second column (the captured group)
result <- match_matrix[, 2]
print(result)
# Output: [1] "1400"

Breakdown:

• ^.*?_.*?_: Matches all text from the start of the string up to (and including) the second underscore.
• ([0-9.]+): Captures one or more digits or decimal points—this is our target value.
• str_match returns a matrix where the first column is the full matched text, and subsequent columns are the captured groups. We take the second column to get just the digits.

内容的提问来源于stack exchange，提问作者Alexander