Regular Expressions 101

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  • PCRE2 (PHP >=7.3)
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/
(?<=[\w ])(?=[\w ]$)[\w ]
/
gm
Positive Lookbehind
(?<=[\w ])
Assert that the Regex below matches
Match a single character present in the list below
[\w ]
{4} matches the previous token exactly 4 times
\w matches any word character (equivalent to [a-zA-Z0-9_])
matches the character literally (case sensitive)
Positive Lookahead
(?=[\w ]$)
Assert that the Regex below matches
Match a single character present in the list below
[\w ]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
\w matches any word character (equivalent to [a-zA-Z0-9_])
matches the character literally (case sensitive)
$ asserts position at the end of a line
Match a single character present in the list below
[\w ]
\w matches any word character (equivalent to [a-zA-Z0-9_])
matches the character 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

Substitution

Processing...