Regular Expressions 101

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/
^apple-touch-icon-(precomposed|[0-9]{2,4}x[0-9]{2,4})?\.png
/
gm
^ asserts position at start of a line
apple-touch-icon-
matches the characters apple-touch-icon- literally (case sensitive)
1st Capturing Group
(precomposed|[0-9]{2,4}x[0-9]{2,4})?
? matches the previous token between zero and one times, as many times as possible, giving back as needed (greedy)
1st Alternative
precomposed
precomposed
matches the characters precomposed literally (case sensitive)
2nd Alternative
[0-9]{2,4}x[0-9]{2,4}
Match a single character present in the list below
[0-9]
{2,4} matches the previous token between 2 and 4 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
x matches the character x with index 12010 (7816 or 1708) literally (case sensitive)
Match a single character present in the list below
[0-9]
{2,4} matches the previous token between 2 and 4 times, as many times as possible, giving back as needed (greedy)
0-9 matches a single character in the range between 0 (index 48) and 9 (index 57) (case sensitive)
\. matches the character . with index 4610 (2E16 or 568) literally (case sensitive)
png
matches the characters png 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