1. Numbers only
Match a string containing only one or more digits.
Should match: 12345
Should reject: A123, 12-34
Show solution
^\d+$
Anchors force the whole value to be digits.
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Regex Exercises: 18 Practice Problems with Solutions
Practice regular expressions from basic anchors and quantifiers to groups, backreferences, lookahead, and lookbehind. Try each problem before opening the solution.
Best way to practice: write your pattern in the Regex Tester, include examples that should fail, and only reveal the solution after you have tested your own idea.
18 problems
Beginner → Intermediate → Advanced
These practice questions are designed to build one concept at a time and then combine concepts in the final problems.
2. Starts with DEV-
Match DEV- followed by exactly four digits.
Should match: DEV-2026
Should reject: DEV-26, XDEV-2026
Show solution
^DEV-\d{4}$Literal text plus an exact digit quantifier.
3. Three-letter file extension
Match file names ending in .jpg, .png, or .gif, ignoring case.
Should match: photo.JPG
Should reject: photo.pdf
Show solution
\.(?:jpg|png|gif)$
Escape the dot and anchor the extension at the end.
4. Letters only
Match ASCII letters only.
Should match: RegexArena
Should reject: Regex2026
Show solution
^[A-Za-z]+$
A character class plus anchors validates the entire value.
5. Optional color spelling
Match both color and colour.
Should match: color, colour
Should reject: colouur
Show solution
^colou?r$
The question mark makes the preceding u optional.
6. Exactly two letters and two digits
Match AB12-style codes.
Should match: AB12
Should reject: A123, ab12
Show solution
^[A-Z]{2}\d{2}$Use exact quantifiers for the two sections.
7. Date format
Match DD/MM/YYYY format only.
Should match: 24/09/2026
Should reject: 2026-09-24
Show solution
^\d{2}/\d{2}/\d{4}$This checks format, not whether the calendar date exists.
8. Simple email shape
Match a practical email-shaped value.
Should match: dev@example.com
Should reject: dev@site, @example.com
Show solution
^[\w.-]+@[\w.-]+\.[A-Za-z]{2,}$Useful for practice, but not a complete implementation of email standards.
9. HTTP or HTTPS URL
Match a simple http/https URL with an optional path.
Should match: https://regexarena.com/en/
Should reject: ftp://example.com
Show solution
^https?://[\w.-]+\.[A-Za-z]{2,}(?:/.*)?$The s is optional; the path group is optional.
10. Log level and date
Match lines beginning with INFO, WARN, or ERROR followed by YYYY-MM-DD.
Should match: ERROR 2026-09-24 Failed
Should reject: DEBUG 2026-09-24 Trace
Show solution
^(INFO|WARN|ERROR)\s+\d{4}-\d{2}-\d{2}Alternation captures the allowed log level.
11. Extract hashtags
Find every hashtag made of word characters.
Should match: #regex #javascript
Should reject: plain text
Show solution
#\w+
Use the global flag in JavaScript to collect all occurrences.
12. Collapse repeated whitespace
Find one or more whitespace characters for replacement.
Should match: a b
Should reject: ab
Show solution
\s+
Replace each match with a single space.
13. Positive lookahead
Capture DEV plus three digits only when it is immediately followed by -OK.
Should match: DEV123-OK
Should reject: DEV123-NO
Show solution
^DEV\d{3}(?=-OK$)The lookahead checks context without consuming -OK.
14. Negative lookahead
Match lowercase names except the exact word admin.
Should match: marcello, regexarena
Should reject: admin
Show solution
^(?!admin$)[a-z]+$
The negative lookahead rejects one reserved value.
15. Positive lookbehind
Capture digits only when preceded by R$ and a space.
Should match: R$ 150
Should reject: USD 150
Show solution
(?<=R\$ )\d+$
The lookbehind checks the prefix without including it in the match.
16. Named capture group
Capture a code such as DEV-2026 in a named group called code.
Should match: DEV-2026
Should reject: DE-2026
Show solution
(?<code>[A-Z]{3}-\d{4})Named groups make extracted data easier to understand.
17. Repeated words
Find a word repeated immediately after itself.
Should match: the the
Should reject: the regex
Show solution
\b(\w+)\s+\1\b
The backreference \1 repeats the text captured by group 1.
18. Context window
Capture AAA-0000 only after ID: and before ;OK.
Should match: ID:DEV-2026;OK
Should reject: ID:DEV-2026;FAIL
Show solution
(?<=ID:)(?<code>[A-Z]{3}-\d{4})(?=;OK$)Combine lookbehind, a named group, and lookahead.
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