Regex Exercises in R: 40 Practice Problems

Forty practice problems on regular expressions in R: anchors, character classes, quantifiers, groups, lookarounds. Solutions hidden.

By Selva Prabhakaran · Published May 11, 2026 · Last updated May 11, 2026

RRun this once before any exercise

library(stringr)

Section 1. Anchors and basics (8 problems)

Exercise 1.1: Starts with

Difficulty: Beginner. Strings starting with "Mr".

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RInteractive R

str_detect(c("Mr Smith","Dr Jones","Mrs Park"), "^Mr ")

Exercise 1.2: Ends with

Difficulty: Beginner. ".csv" filenames.

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RInteractive R

str_detect(c("a.csv","b.txt","c.csv"), "\\.csv$")

Exercise 1.3: Contains digit

Difficulty: Beginner.

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RInteractive R

str_detect(c("abc","abc123","xy"), "\\d")

Exercise 1.4: Word boundary

Difficulty: Intermediate.

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RInteractive R

str_detect(c("cat","catalog","scat"), "\\bcat\\b")

Exercise 1.5: Whole-string match

Difficulty: Intermediate. Exactly 5 digits.

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RInteractive R

str_detect(c("12345","1234","12345abc"), "^\\d{5}$")

Exercise 1.6: Empty string

Difficulty: Beginner.

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RInteractive R

str_detect(c("", "x"), "^$")

Exercise 1.7: Whitespace-only

Difficulty: Intermediate.

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RInteractive R

str_detect(c(" ","abc","\t\n"), "^\\s*$")

Exercise 1.8: Case-insensitive

Difficulty: Intermediate.

Show solution

RInteractive R

str_detect(c("Error","ERROR","ok"), regex("error", ignore_case = TRUE))

Section 2. Character classes and quantifiers (8 problems)

Exercise 2.1: Letters only

Difficulty: Beginner.

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RInteractive R

str_detect(c("abc","abc1","123"), "^[A-Za-z]+$")

Exercise 2.2: Digits only

Difficulty: Beginner.

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RInteractive R

str_detect(c("123","12a","a1"), "^\\d+$")

Exercise 2.3: Alphanumeric

Difficulty: Beginner.

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RInteractive R

str_detect(c("abc123","abc!"), "^[A-Za-z0-9]+$")

Exercise 2.4: Hex string

Difficulty: Intermediate.

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RInteractive R

str_detect(c("ff00aa","gg11","123abc"), "^[0-9a-fA-F]+$")

Exercise 2.5: Min length 8

Difficulty: Intermediate.

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RInteractive R

str_detect(c("short","longer_one"), "^.{8,}$")

Exercise 2.6: Exact length

Difficulty: Intermediate. 5 digits.

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RInteractive R

str_detect(c("12345","123","123456"), "^\\d{5}$")

Exercise 2.7: Range length

Difficulty: Intermediate. 3 to 6 letters.

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RInteractive R

str_detect(c("ab","abc","abcdefg"), "^[a-zA-Z]{3,6}$")

Exercise 2.8: Optional group

Difficulty: Advanced. Optional "+1" prefix on phones.

Show solution

RInteractive R

str_detect(c("+15555550100","5555550100"), "^(\\+1)?\\d{10}$")

Section 3. Groups and capturing (8 problems)

Exercise 3.1: Capture domain

Difficulty: Intermediate.

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RInteractive R

str_match("a@x.com", "@(.+)$")

Exercise 3.2: Multiple groups

Difficulty: Intermediate.

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RInteractive R

str_match("user_42", "(\\w+)_(\\d+)")

Exercise 3.3: Named groups

Difficulty: Advanced.

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RInteractive R

str_match("John 30", "(?<name>\\w+) (?<age>\\d+)")

Exercise 3.4: Backreference in replacement

Difficulty: Advanced. Swap "John Smith" -> "Smith, John".

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RInteractive R

str_replace("John Smith", "(\\w+) (\\w+)", "\\2, \\1")

Exercise 3.5: Non-capturing group

Difficulty: Advanced.

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RInteractive R

str_match("color", "colou?r") str_match("color", "colo(?:u)?r")

Exercise 3.6: Alternation in group

Difficulty: Intermediate.

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RInteractive R

str_detect(c("cat","dog","fish"), "^(cat|dog)$")

Exercise 3.7: Repeated group

Difficulty: Advanced. Match repeated digits like "123-456-7890".

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RInteractive R

str_detect("123-456-7890", "^(\\d{3}-){2}\\d{4}$")

Exercise 3.8: Match all groups

Difficulty: Advanced.

Show solution

RInteractive R

str_match_all("user_1 user_2 user_3", "user_(\\d+)")

Section 4. Lookarounds (6 problems)

Exercise 4.1: Lookbehind

Difficulty: Advanced. Digit after "$".

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RInteractive R

str_extract("Total: $100", "(?<=\\$)\\d+")

Exercise 4.2: Negative lookbehind

Difficulty: Advanced.

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RInteractive R

str_extract_all("Item 5, $10 each", "(?<!\\$)\\d+")

Exercise 4.3: Lookahead

Difficulty: Advanced. Digit BEFORE "px".

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RInteractive R

str_extract("font: 14px", "\\d+(?=px)")

Exercise 4.4: Negative lookahead

Difficulty: Advanced. Numbers NOT followed by "px".

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RInteractive R

str_extract_all("a:14px b:7em c:3", "\\d+(?!px)")

Exercise 4.5: Combined

Difficulty: Advanced.

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RInteractive R

str_extract("between [BEGIN] foo [END]", "(?<=\\[BEGIN\\]).+?(?=\\[END\\])")

Exercise 4.6: Password rule

Difficulty: Advanced. At least one digit, one letter, length 8+.

Show solution

RInteractive R

str_detect(c("abc12345","abcdefgh","12345678"), "^(?=.*[A-Za-z])(?=.*\\d).{8,}$")

Section 5. Real-world (10 problems)

Exercise 5.1: Email

Difficulty: Intermediate.

Show solution

RInteractive R

str_detect(c("a@x.com","not_email","b@y.co.uk"), "^[\\w.]+@[\\w.]+\\.\\w{2,}$")

Exercise 5.2: URL

Difficulty: Intermediate.

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RInteractive R

str_detect("https://r-statistics.co", "^https?://[\\w.-]+(/.*)?$")

Exercise 5.3: ISO date

Difficulty: Intermediate.

Show solution

RInteractive R

str_detect(c("2024-01-15","2024/01/15"), "^\\d{4}-\\d{2}-\\d{2}$")

Exercise 5.4: Phone US

Difficulty: Intermediate.

Show solution

RInteractive R

str_detect("(415) 555-1234", "^\$\\d{3}\$ \\d{3}-\\d{4}$")

Exercise 5.5: Hashtag extraction

Difficulty: Intermediate.

Show solution

RInteractive R

str_extract_all("Loving #rstats and #dataviz", "#\\w+")

Exercise 5.6: Strip HTML tags

Difficulty: Advanced.

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RInteractive R

str_replace_all("<p>Hello <b>world</b></p>", "<[^>]+>", "")

Exercise 5.7: Mask credit card

Difficulty: Advanced.

Show solution

RInteractive R

str_replace_all("CC: 4111-1111-1111-1234", "\\d{4}-\\d{4}-\\d{4}-\\d{4}", "XXXX-XXXX-XXXX-XXXX")

Exercise 5.8: Extract numbers from messy text

Difficulty: Intermediate.

Show solution

RInteractive R

str_extract_all("p1=10, p2=20.5, p3=300", "[-+]?\\d*\\.?\\d+")

Exercise 5.9: First word

Difficulty: Intermediate.

Show solution

RInteractive R

str_extract("Hello World Today", "^\\S+")

Exercise 5.10: Detect repeated words

Difficulty: Advanced. e.g. "the the".

Show solution

RInteractive R

str_detect("the the cat sat", "\\b(\\w+)\\b\\s+\\1\\b")

What to do next

stringr-Exercises (shipped), string-tooling drills.
Data-Cleaning-Exercises (shipped), regex inside cleanup.