Regex exactly n OR m times
Match a sequence n or m times in Java regex with (?:(pattern){n})|(?:(pattern){m}). For instance, if you need 'a' thrice or five times, your regex would look like this:
The ins and outs of regex quantification
In regex, quantifiers govern how often a sequence appears. Here are the basic ones:
X?denotes 0 or 1 occurrence of X (essentially, X is optional)X*means 0 or more occurrences of X (X may or may not appear)X+signifies 1 or more occurrences of X (X must be there, but may repeat)X{n}signals exactly n occurrences of X (X must appear n times – no more, no less)
But your task is to match a sequence exactly n or m times, so we need to get crafty and blend regex patterns accordingly.
The art of advanced pattern crafting
If we have two discrete numbers, 3 and 5 for instance, and we need to match either 3 or 5 occurrences of a pattern 'X', we need to structure our regex to cater to both, excluding any number in between.
Ordering for efficiency
Place the most frequent pattern first as regex engines match from left to right – it's their polite way of saying "First come, first served." This can make your regex faster, especially if you're analyzing War and Peace out there.
Dodging the redundancy bullet with lazy quantifiers
Add lazy quantifiers or, in the most stubborn cases, use atomic groups to tell your regex engine to stop beating around the bush and cut to the chase. This helps avoid unnecessary backtracking, which could slow your code down like molasses in January.
The joy of non-capturing groups
Unless you're saving those precious matched groups for later, use the more streamlined non-capturing groups (?: ... ). This tip keeps your engine's memory light and your code fast, without any information overload.
Precision matching with lookaheads and lookbehinds
When your matches need to be as precise as a Swiss watch, consider positive lookaheads (?= ... ) or lookbehinds (?<= ... ). They'll only match when your criteria are met exactly, no compromises.
Optimization tips and troubleshooting
Mastering regex for matching n or m occurrences means you need to not only get it right, but you also want to run it efficiently, especially if your text looks like it swallowed the Lord of the Rings trilogy.
Setting boundaries
Before you set out on your quest, ensure you've got your line starts (^) and ends ($) sorted. Nothing's more annoying than snagging a 4-scoop scoop when you explicitly specified 3 or 5!
Invoking the powers of regex tools
Many programs struggle with regex – but not sites like regex101.com. Valuable for test drives and debugging, these tools give on-spot feedback, explanations, and even a performance rating. Ideal when you're chasing that last gremlin out of your code!
The virtue of patience and experimentation
Regex can be more art than science sometimes, so don't be afraid to experiment. Play around with different combinations, and who knows, you might stumble upon the most artful, elegant solution anyone has ever seen.
Special note: when m equals 2n
If you find yourself in the strange phenomenon where m equals 2n (or some other integral multiple), you can use patterns like this:
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