/ ^ ([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) | ([a-zA-Z]) $/ .test ("11a") / / true
and
/ ^ (([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) | ([a-zA-Z])) $/ .test ("11a") / / false
Why are the results different
The difference between the two regular expressions is that I added a parenthesis to the outermost layerregular matching results are different. There is no reason why. You said that you added parentheses, you should understand the meaning of parentheses: group the matching results
Let's first explain the meaning of regularity.
decompose
/ ^ ([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) | ([a-zA-Z]) $/
^ ([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) | ([a-zA-Z])
enter a string The starting match of([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9])or([a-zA-Z])one of the conditions
([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) | ([a-zA-Z]) $
the end position of the input string matches fully([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9])or([a-zA-Z])one of the conditions
be more specific
[a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]match for a-zA-Z once | match for [A-Za-z0-9] 0 times | | multiple times
[a-zA-Z]this means a-zA-z doesn't explain much
()match the result and get it
if you add () matching result to it, then the result of or (which can be understood as "or") is left regular [a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9] before the matching result is true !
I know I'm late, but can I just explain it to you?
begins with ^ and ends with $ has higher priority than | , so the first example is actually | two separate segments ^ ([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) and ([a-zA-Z]) $.
you can write down this priority
\
(), (?:), (?=), []
*, +, ?, {n}, {n,}, {n,m}
^, $, \ :
| ""
Let me join in the fun:
Let's first take a look at
/ ^ ([a-zA-Z] + [A-Za-z0-9 -] * [a-zA-Z0-9]) | ([a-zA-Z]) $/ the meaning of the regular expression
below is the regular railway diagram
group -sharp1group -sharp2
"11a" group -sharp2
/^(([a-zA-Z]+[A-Za-z0-9-]*[a-zA-Z0-9])|([a-zA-Z]))$/
group -sharp1group -sharp1
group -sharp1
group -sharp2 group -sharp3 group -sharp2 group -sharp3
group -sharp2 "[1-] ||[0-] |[1]"
group -sharp3 "[1]"
group -sharp2 "11a" group2 , group -sharp3a
.
but group-sharp3 only hits the last trailing character, not the first character 1, so the whole match is not matched.
I don't know if this answer is clear enough.
Let me give a simple and rude explanation.
the first indicates that it begins with (group1) or ends with (group2), and the second represents
that begins with (group1) or begins with (group2).
Please take a look at the following case:
D:\temp>node test.js
1 true true
2 true true
12 true false
10 true false
02 true false
you can see that reg1 represents the entire string starts with 1 or ends with 2 , and reg2 indicates that the entire string can only be 1 or 2. It can be understood as | has lower priority than $ and ^ .
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