Regular Expressions 101

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  • PCRE2 (PHP >=7.3)
  • PCRE (PHP <7.3)
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/
(?<=[\s>])#([\w_]+)\b(?!;)
/
g
Positive Lookbehind
(?<=[\s>])
Assert that the Regex below matches
Match a single character present in the list below
[\s>]
\s matches any whitespace character (equal to [\r\n\t\f\v ])
> matches the character > literally (case sensitive)
# matches the character # literally (case sensitive)
1st Capturing Group
([\w_]+)
Match a single character present in the list below
[\w_]+
+ Quantifier — Matches between one and unlimited times, as many times as possible, giving back as needed (greedy)
\w matches any word character (equal to [a-zA-Z0-9_])
_ matches the character _ literally (case sensitive)
\b assert position at a word boundary: (^\\w|\\w$|\\W\\w|\\w\\W)
Negative Lookahead
(?!;)
Assert that the Regex below does not match
; matches the character ; literally (case sensitive)
Global pattern flags
g modifier: global. All matches (don't return after first match)
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
g

Test String