在R语言中如何用正则表达式匹配斜体与下划线格式内容

Hey there! Let's tackle your regex questions for R step by step.

Your first goal is to match the 1st and 3rd strings in your vector, which contain <em> opening and closing tags.

The regex pattern targets any string that includes at least one pair of <em> and </em> tags (with any content in between, plus any text before or after the tags). Here's the pattern and how to use it in R:

# Define the regex pattern
em_pattern <- "^.*<em>.*</em>.*$"

# Your original vector
ex <- c("what <em>did</em> you do?", "i don鈥檛 know", "<em>Pe</em>ter is angry.")

# Check matches with grepl()
grepl(em_pattern, ex)
# Output: TRUE FALSE TRUE

Breakdown of the pattern: #

• ^ anchors the match to the start of the string
• .* matches any character (except newlines) zero or more times
• <em> and </em> match the literal HTML tags exactly
• $ anchors the match to the end of the string

If the italicized content uses underscores instead (e.g., what _did_ you do? and _Pe_ter is angry.), we adjust the regex to target underscore-wrapped content.

For precise matching (ensuring there's actual text between the underscores), use this pattern:

# Define the underscore regex pattern
underscore_pattern <- "^.*_[^_]+_.*$"

# Updated vector with underscore formatting
ex_underscore <- c("what _did_ you do?", "i don鈥檛 know", "_Pe_ter is angry.")

# Check matches
grepl(underscore_pattern, ex_underscore)
# Output: TRUE FALSE TRUE

Breakdown of the pattern: #

• _ matches the literal underscore character
• [^_]+ matches one or more characters that are NOT underscores (prevents matching empty __ pairs)
• ^.* and .*$ handle any text before or after the underscore-wrapped content

If you don't need to worry about empty underscore pairs, you could simplify the pattern to ^.*_.*_.*$—it works for your specific example, but the [^_]+ version is more robust for real-world use.

内容的提问来源于stack exchange，提问作者Chris Ruehlemann