Regular Expressions 101

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/
(?<=[a-z])(?=[A-Z])|(?<=[A-Z])(?=[A-Z][a-z])
/
gm
1st Alternative
(?<=[a-z])(?=[A-Z])
Positive Lookbehind
(?<=[a-z])
Assert that the Regex below matches
Match a single character present in the list below
[a-z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
Positive Lookahead
(?=[A-Z])
Assert that the Regex below matches
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
2nd Alternative
(?<=[A-Z])(?=[A-Z][a-z])
Positive Lookbehind
(?<=[A-Z])
Assert that the Regex below matches
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
Positive Lookahead
(?=[A-Z][a-z])
Assert that the Regex below matches
Match a single character present in the list below
[A-Z]
A-Z matches a single character in the range between A (index 65) and Z (index 90) (case sensitive)
Match a single character present in the list below
[a-z]
a-z matches a single character in the range between a (index 97) and z (index 122) (case sensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
m modifier: multi line. Causes ^ and $ to match the begin/end of each line (not only begin/end of string)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gm

Test String