Regular Expressions 101

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/
\b(\w+\.(png|jpeg|gif|jpg))\b
/
gm
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
1st Capturing Group
(\w+\.(png|jpeg|gif|jpg))
\w
matches any word character (equivalent to [a-zA-Z0-9_])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
2nd Capturing Group
(png|jpeg|gif|jpg)
1st Alternative
png
png
matches the characters png literally (case sensitive)
2nd Alternative
jpeg
jpeg
matches the characters jpeg literally (case sensitive)
3rd Alternative
gif
gif
matches the characters gif literally (case sensitive)
4th Alternative
jpg
jpg
matches the characters jpg literally (case sensitive)
\b assert position at a word boundary: (^\w|\w$|\W\w|\w\W)
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