Regular Expressions 101

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/
(?:(?<=\h)|^)(\w*foo\w*)(?=\h|$)
/
gm
Non-capturing group
(?:(?<=\h)|^)
1st Alternative
(?<=\h)
Positive Lookbehind
(?<=\h)
Assert that the Regex below matches
\h matches any horizontal whitespace character (equivalent to [[:blank:]])
2nd Alternative
^
^ asserts position at start of a line
1st Capturing Group
(\w*foo\w*)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
foo
matches the characters foo literally (case sensitive)
\w
matches any word character (equivalent to [a-zA-Z0-9_])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
Positive Lookahead
(?=\h|$)
Assert that the Regex below matches
1st Alternative
\h
\h matches any horizontal whitespace character (equivalent to [[:blank:]])
2nd Alternative
$
$ 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