regex101: PCRE (Legacy) Substitution Workspace

Explanation

Pro

Group 1((?:(?:https?|ftp):\/{2})(?:(?:[0-9a-z_@-]+\.)+(?:[0-9a-z]){2,4})?(?:(?:\/(?:[~0-9a-z\#\+\%\@\.\/_-]+))?\/)*(?=\s|\"))

Non-Capturing Group(?:(?:https?|ftp):\/{2})

Non-Capturing Group(?:https?|ftp)

1st Alternativehttps?

httpthe literal text http (case insensitive)

s?optionally matches the literal s (115₁₀ / 163₈ / 73₁₆) (case insensitive)

2nd Alternativeftp

ftpthe literal text ftp (case insensitive)

:the literal : (58₁₀ / 72₈ / 3A₁₆)

\/{2}the literal / (47₁₀ / 57₈ / 2F₁₆) , repeated exactly 2 times

Non-Capturing Group(?:(?:[0-9a-z_@-]+\.)+(?:[0-9a-z]){2,4})?

?repeats the group contents below at most once:

Non-Capturing Group(?:[0-9a-z_@-]+\.)+

+repeats the group contents below at least once:

Character Class[0-9a-z_@-]+

+matches at least one character from the set below:

0-9one character from 0 (48₁₀) to 9 (57₁₀)

a-zone character from a (97₁₀) to z (122₁₀) (case insensitive)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

@the literal @ (64₁₀ / 100₈ / 40₁₆)

-the literal - (45₁₀ / 55₈ / 2D₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

Non-Capturing Group(?:[0-9a-z]){2,4}

{2,4}repeats the group contents below between 2 and 4 times:

Character Class[0-9a-z]

0-9one character from 0 (48₁₀) to 9 (57₁₀)

a-zone character from a (97₁₀) to z (122₁₀) (case insensitive)

Non-Capturing Group(?:(?:\/(?:[~0-9a-z\#\+\%\@\.\/_-]+))?\/)*

*repeats the group contents below any number of times:

Non-Capturing Group(?:\/(?:[~0-9a-z\#\+\%\@\.\/_-]+))?

?repeats the group contents below at most once:

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

Non-Capturing Group(?:[~0-9a-z\#\+\%\@\.\/_-]+)

Character Class[~0-9a-z\#\+\%\@\.\/_-]+

+matches at least one character from the set below:

~the literal ~ (126₁₀ / 176₈ / 7E₁₆)

0-9one character from 0 (48₁₀) to 9 (57₁₀)

a-zone character from a (97₁₀) to z (122₁₀) (case insensitive)

\#the literal # (35₁₀ / 43₈ / 23₁₆)

\+the literal + (43₁₀ / 53₈ / 2B₁₆)

\%the literal % (37₁₀ / 45₈ / 25₁₆)

\@the literal @ (64₁₀ / 100₈ / 40₁₆)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

_the literal _ (95₁₀ / 137₈ / 5F₁₆)

-the literal - (45₁₀ / 55₈ / 2D₁₆)

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

Positive Lookahead(?=\s|\")

1st Alternative\s

\sany whitespace character

2nd Alternative\"

\"the literal " (34₁₀ / 42₈ / 22₁₆)

Group 2(\/)?

?repeats the group contents below at most once:

\/the literal / (47₁₀ / 57₈ / 2F₁₆)

Group 3(index\.html?)?

?repeats the group contents below at most once:

indexthe literal text index (case insensitive)

\.the literal . (46₁₀ / 56₈ / 2E₁₆)

htmthe literal text htm (case insensitive)

l?optionally matches the literal l (108₁₀ / 154₈ / 6C₁₆) (case insensitive)

Global pattern flags

g modifier: global. Finds all matches instead of stopping after the first

i modifier: insensitive. Matches without distinguishing uppercase and lowercase letters

Quick Reference

25 tokens

Regular Expression

((?:(?:https?|ftp):\/{2})(?:(?:[0-9a-z_@-]+\.)+(?:[0-9a-z]){2,4})?(?:(?:\/(?:[~0-9a-z\#\+\%\@\.\/_-]+))?\/)*(?=\s|\"))(\/)?(index\.html?)?

New

there is some text with <a href=" http://www.Something.com/ "> but without index.html; then there is another one that is <a href = "https://www.other.com/more/stuff/index.html"> which should be unaltered.
Finally does it work for href = "http://www.com/stuff/"
http://www.index.com/
http://ex.com/a/b/"

http://www.example.com                       add /index.html
http://www.example.com/something             do nothing
http://www.example.com/something/            add /index.html 
http://www.example.com/something/index.html  do nothing

Substitution

\1/index.html

there is some text with <a href=" http://www.Something.com/ "> but without index.html; then there is another one that is <a href = "https://www.other.com/more/stuff/index.html"> which should be unaltered.
Finally does it work for href = "http://www.com/stuff/"
http://www.index.com/
http://ex.com/a/b/"

http://www.example.com                       add /index.html
http://www.example.com/something             do nothing
http://www.example.com/something/            add /index.html 
http://www.example.com/something/index.html  do nothing