I am trying to do the following:

Take a string abcdEFGpcf and replace whatever is between abcd and pcf by something that depends on EFG, for example by EFGH and then obtain abcdEFGHpcf. I am trying to use regex for this but I'm extremely confused by the notation.

I guess it's also possible to do this with string patterns but I am also confused by the notation. I know how to replace a fixed substring by something else, but in this case we need to reference $x=$ EFG and put it in the new expression.

  • $\begingroup$ Do you mean something like: StringReplace["abcdEFGpcf", "abcd" ~~ x__ ~~ "pcf" :> "abcd" ~~ ToLowerCase[x] ~~ "pcf"] ? $\endgroup$
    – Domen
    Commented May 12, 2023 at 19:27
  • $\begingroup$ @Domen Quite there. I was looking for StringReplace["abcdEFGpcf", "abcd" ~~ x__ ~~ "pcf" :> "abcd" ~~ x ~~ "H" ~~ "pcf"] but your code helped me a lot. And I guess now I solved this doubt and all further doubts I had. Thank you so much! $\endgroup$
    – Red Banana
    Commented May 12, 2023 at 19:29

2 Answers 2


Let's put your transformation into a function for re-usability:

func[str_] := str <> "H"

Here's your input:

input = "abcdEFGpcf"

Now, we can use either string expressions or RegularExpression:

StringReplace[input, head : "abcd" ~~ mid___ ~~ tail : "pcf" :> head <> func[mid] <> tail]


In the above, I named the two constant patterns to avoid repeating them in the rule.

StringReplace[input, RegularExpression["(abcd)(.*)(pcf)"] -> "$1" <> func["$2"] <> "$3"]


In this case, we don't need to explicitly name the patterns, because we can use $n on the right hand side to reference a captured substring.


You could also just use StringReplacePart, no need for patterns !

s = "abcdYYYYpcfabcdXXXXpcf"
a = "abcd"
b = "pcf"
rep = "EFGH"

In the above s is your string. a and b are the first and last terminal strings, and you want to replace everything in between them with rep. Right? Then

p1 = StringPosition[s, a]
p2 = StringPosition[s, b]
m = MapThread[{#1[[2]] + 1, #2[[1]] - 1} &, {p1, p2}]

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The above gives the positions within s with each starting and ending terminal strings are located. Now use

StringReplacePart[s, rep, m]

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ps. I see now you want replacement to happen only if string inside these terminal strings depends "EFG". The above code can be modified to do that, by adding a check inside the MapThread to only pick these positions if the inner string satisfies this conditions.

This version only does the replacement if the inner string contains "EFG"

          result = StringReplacePart[s,replS,m]

Example use:

replaceSubStrings["abcdEFGpcfabcdXXXXpcf", "abcd", "pcf", "EFGH", "EFG"]

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 replaceSubStrings["abcdxxxpcf", "abcd", "pcf", "EFGH", "EFG"]

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