diff --git a/src/util/regexp.ts b/src/util/regexp.ts
new file mode 100644
index 0000000..c01f820
--- /dev/null
+++ b/src/util/regexp.ts
@@ -0,0 +1,26 @@
+export type SubstitutionCallable = (match: RegExpExecArray|null) => string;
+
+
+/**
+ * Return the string obtained by replacing the leftmost non-overlapping
+ * occurrences of the pattern in `input` by the `substitution`. `substitution`
+ * can be either a string or a callable; if a string, backslash escapes in it
+ * are processed. If it is a callable, it's passed the `RegExpExecArray` object
+ * and must return a substitution string to be used.
+ */
+export function substitute(
+    pattern: RegExp,
+    input: string,
+    substitution: string|SubstitutionCallable,
+): string {
+    return input.replace(pattern, (match, ...groups) => {
+        const execArray = pattern.exec(match);
+
+        if (typeof substitution === "function") return substitution(execArray);
+
+        return substitution.replace(
+            /\\(\d+)/g,
+            (_, index) => execArray?.[Number(index)] ?? ''
+        );
+    });
+}