Regular Expressions 101

Save & Share

  • Save Regex
    ctrl+s
  • Update Regex
    ctrl+⇧+s

Flavor

  • PCRE2 (PHP >=7.3)
  • PCRE (PHP <7.3)
  • ECMAScript (JavaScript)
  • Python
  • Golang
  • Java 8

Function

  • Match
  • Substitution
  • List
  • Unit Tests
/
(<link\s+[^>]*rel\s*=\s*(['"])stylesheet\2.*?>)(?:(?=.*<\/head))
/
gmsi
1st Capturing Group
(<link\s+[^>]*rel\s*=\s*(['"])stylesheet\2.*?>)
<link
matches the characters <link literally (case insensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
+ matches the previous token between one and unlimited times, as many times as possible, giving back as needed (greedy)
Match a single character not present in the list below
[^>]
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
> matches the character > with index 6210 (3E16 or 768) literally (case insensitive)
rel
matches the characters rel literally (case insensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
= matches the character = with index 6110 (3D16 or 758) literally (case insensitive)
\s
matches any whitespace character (equivalent to [\r\n\t\f\v ])
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
2nd Capturing Group
(['"])
Match a single character present in the list below
['"]
'"
matches a single character in the list '" (case insensitive)
stylesheet
matches the characters stylesheet literally (case insensitive)
\2 matches the same text as most recently matched by the 2nd capturing group
.
matches any character
*? matches the previous token between zero and unlimited times, as few times as possible, expanding as needed (lazy)
> matches the character > with index 6210 (3E16 or 768) literally (case insensitive)
Non-capturing group
(?:(?=.*<\/head))
Positive Lookahead
(?=.*<\/head)
Assert that the Regex below matches
.
matches any character
* matches the previous token between zero and unlimited times, as many times as possible, giving back as needed (greedy)
< matches the character < with index 6010 (3C16 or 748) literally (case insensitive)
\/ matches the character / with index 4710 (2F16 or 578) literally (case insensitive)
head
matches the characters head literally (case insensitive)
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)
s modifier: single line. Dot matches newline characters
i modifier: insensitive. Case insensitive match (ignores case of [a-zA-Z])
Your regular expression does not match the subject string.Try launching the debugger to find out why.

Regular Expression
No Match

/
/
gmsi

Test String