Regular Expressions 101

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/
^\/([a-zA-Z]{2}-[a-zA-Z]{2})\/cms\/api\/preview([a-zA-Z0-9-_=\/]*)(\?)(.*)
/
mg
^ asserts position at start of a line
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
1st Capturing Group
([a-zA-Z]{2}-[a-zA-Z]{2})
Match a single character present in the list below
[a-zA-Z]
{2} matches the previous token exactly 2 times
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
Match a single character present in the list below
[a-zA-Z]
{2} matches the previous token exactly 2 times
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
cms
matches the characters cms literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
api
matches the characters api literally (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
preview
matches the characters preview literally (case sensitive)
2nd Capturing Group
([a-zA-Z0-9-_=\/]*)
Match a single character present in the list below
[a-zA-Z0-9-_=\/]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
This hyphen is treated literally, which might be confusing for others. Consider escaping it or placing at the start or end of the class!
_=
matches a single character in the list _= (case sensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case sensitive)
3rd Capturing Group
(\?)
\? matches the character ? with index 6310 (3F16 or 778) literally (case sensitive)
4th Capturing Group
(.*)
.
matches any character (except for line terminators)
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Global pattern flags
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.

Regular Expression
No Match

/
/
mg

Test String