Keentune
Adaptive skill practice — find your level, get sharper.

Regex practice

Predict what a pattern matches
Master regular expressions by predicting what they do — character classes and anchors, quantifiers (greedy vs lazy), capturing groups, the global flag, and the match/replace/split operations. Every answer is computed by running the pattern in a real regex engine (JavaScript flavor, noted in each question), so the explanations are exact. Builds the pattern-reading fluency you need for interviews and everyday work. Part of the Developer track.
• Meets you at your level — never too easy, never too hard.
• Adapts every question to how you’re doing.
• Tracks your level over time. Skill practice, not a test.
New here? Read the Regex guide
A free 18-minute primer — the mental model, the mistakes beginners make, and what to practise first.
Curriculum
213 concepts across 24 chapters, from foundations through advanced practice — each one a concept the adaptive question bank can teach and test.
A. What a regex is and how a match happens
9
B. Literal characters and escaping
8
C. Character classes
10
D. Shorthand class escapes and the wildcard
8
E. Unicode mode, property escapes and set algebra
11
F. Quantifiers and repetition
10
G. Greediness, laziness and possessiveness
9
H. Anchors and input boundaries
7
I. Word boundaries
7
J. Alternation and its ordering semantics
7
K. Groups: capturing, non-capturing and atomic
10
L. Named capturing groups
6
M. Backreferences
7
N. Lookaround
11
O. Flags and inline modifiers
10
P. The backtracking engine
10
Q. Catastrophic backtracking and ReDoS
9
R. Global and sticky state
8
S. Match results and the matching API
10
T. Replacement syntax
10
U. Splitting
7
V. Python's re module in particular
11
W. PCRE-only constructs
9
X. Using regex well, and when not to
9
View the full curriculum →
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