Regular Expressions 101

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/
(?x) ^ ([^-]+) # Capture everything not a dash \s+-\s+ # Match white space(s) dash white space(s) ((?1)) # Capture everything not a dash \.mp3 $
/
gm
(?x)
match the remainder of the pattern with the following effective flags: gmx
x modifier: extended. Spaces and text after a # in the pattern are ignored
^ asserts position at start of a line
1st Capturing Group
([^-]+)
Match a single character not present in the list below
[^-]
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
# Capture everything not a dash
Capture everything not a dash
Comment: Capture everything not a dash
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
- matches the character - with index 4510 (2D16 or 558) literally (case sensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
# Match white space(s) dash white space(s)
Match white space(s) dash white space(s)
Comment: Match white space(s) dash white space(s)
2nd Capturing Group
((?1))
(?1) recurses the 1st subpattern
# Capture everything not a dash
Capture everything not a dash
Comment: Capture everything not a dash
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
mp3
matches the characters mp3 literally (case sensitive)
$ asserts position at the end of a line
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