How to Test for an Expression Occurring Exactly N or M Times Using Regular Expressions?

Testing for X Occurring Exactly n or m Times Using Regular Expressions

In the realm of regular expressions, it is often crucial to match specific patterns of occurrences. One particular scenario is the need to test for an expression X occurring exactly n or m times. While there is no dedicated quantifier for this exact purpose, there are approaches that can effectively achieve this.

Alternative Approaches

Instead of using a single quantifier, you can utilize the following methods:

• Pattern with Quantifiers: Use the pattern X{n}|X{m}, where X represents the expression to be matched. This pattern will test for X occurring exactly n or m times.
• Nested Quantifiers: Consider an alternative form of X{m}(X{k})?, where m is less than n and k is the value of n-m. This pattern matches X occurring m times, followed by an optional occurrence of X occurring k times.

Example

For instance, to test for "foo" occurring exactly 2 or 3 times, you can use:

• foo{2}|foo{3}
• foo{2}(foo)?

Conclusion

Although there is no specific quantifier for matching X exactly n or m times, the methods outlined above provide effective solutions to achieve this functionality using regular expressions.

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