为何re-find返回含两个元素的向量？预期单个值却不符的原因

Let’s tackle your two questions about re-find one by one—this is super common confusion, so you’re not alone!

1. Why does re-find return a vector with two elements? #

The short answer: your regular expression includes a capture group (parentheses), and re-find returns a vector where:

• The first element is the entire substring that matched the full regex pattern
• Each subsequent element is the content matched by each capture group in order

For example:

(re-find #"(\d+)" "My age is 30")
;; Returns ["30" "30"]

Here, the full regex (\d+) matches the substring "30", and the capture group (\d+) also captures "30"—hence the two-element vector. If you had multiple capture groups, you’d get more elements:

(re-find #"(\w+) (\w+)" "John Doe")
;; Returns ["John Doe" "John" "Doe"]

If you don’t want the capture group to add an element to the result, use a non-capturing group (?:...) instead:

(re-find #"(?:\d+)" "My age is 30")
;; Returns "30" (just the full match, no extra vector element)

2. Why does my specific re-find usage return something other than the single value I expected? #

This almost always ties back to how capture groups work (like the first question) or a mismatch between your regex pattern and the target string. Let’s cover the most common scenarios:

Scenario 1: You expected only the capture group value, not the full match #

If you wrote something like:

(re-find #"(\w+)" "Hello World")
;; Returns ["Hello" "Hello"] instead of just "Hello"

You’re seeing the full match plus the capture group. To get just the capture group value, use second to grab the second element of the vector:

(second (re-find #"(\w+)" "Hello World"))
;; Returns "Hello"

Scenario 2: Your regex has unintended capture groups #

Maybe you added parentheses for grouping (like alternation) but forgot they’re capture groups. For example:

(re-find #"cat|dog" "I have a cat")
;; Returns "cat" (single value, no capture groups)
(re-find #"(cat|dog)" "I have a cat")
;; Returns ["cat" "cat"] (unintended capture group adds an element)

Fix this by using non-capturing groups for alternation:

(re-find #"(?:cat|dog)" "I have a cat")
;; Returns "cat" (single value as expected)

Scenario 3: Your regex isn’t matching what you think it is #

Sometimes the issue isn’t re-find itself—it’s that your regex pattern doesn’t align with the target string. For example, if you use anchors like ^ (start of string) or $ (end of string) incorrectly:

(re-find #"^cat" "I have a cat")
;; Returns nil (because "cat" isn’t at the start of the string)

Double-check your regex to make sure it’s targeting the substring you actually want to match.

内容的提问来源于stack exchange，提问作者THX1137