Regex expression for 2 identical strings in a row - regex

Question

So I am trying to create a regex expression for the following template.
"[alphaNumeric]String/String.xcl"
So
[a1B2c3]Hello/Hello.xcl would pass
a1B2c3]hello/hello.xcl fails
[a1B2c3]Hello/hello.xcl fails
[a1B2c3]hello/hello.xc fails
I have tried the following so far:
\[[\da-zA-Z]+\][a-z]+\/[a-z]+\.xcl$
How do I check if the middle strings are identical?

Answer 1

Use a backreference:
\[[a-zA-Z0-9]+\]([^/]+)/\1\.xcl
The term in parenthesis captures the first part of your path. We may then refer to it later in the regex using \1.
Depending on how you plan to use this regex, you might need optional starting and closing anchors (^ and $).
Demo

Answer 2

You may capture the part after brackets and use a backreference after /:
^\[[\da-zA-Z]+]([A-Za-z]+)\/\1\.xcl$
^^^^^^^^^^ ^^
See the regex demo
Details
^ - start of the string
\[ - a [
[\da-zA-Z]+ - 1+ alphanumeric chars
] - a ] char
([A-Za-z]+) - Capturing group 1: one or more letters
\/ - a slash
\1 - a backreference to capturing group 1 value
\.xcl - .xcl substring
$ - end of string.
NOTE: If you do not care about what kind of chars there can be inside brackets, you may replace [\da-zA-Z]+ with [^\]]+.
NOTE2: If you want to match any chars on both ends of /, replace ([A-Za-z]+) with ([^\/]+).