Regular Expressions 101

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/
(?<=_)(\d+)(?=\.jpg)
/
gm
Positive Lookbehind
(?<=_)
Assert that the Regex below matches
_ matches the character _ with index 9510 (5F16 or 1378) literally (case sensitive)
1st Capturing Group
(\d+)
\d
matches a digit (equivalent to [0-9])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Positive Lookahead
(?=\.jpg)
Assert that the Regex below matches
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
jpg
matches the characters jpg literally (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

Regex Debugger

Please wait while your expression is being debugged...