Error on `$<num><non_num>` capture replacement names (#258)1.0.0-pre-alpha.51.0.0-alpha.0

* Mostly working capture name validation

* Improve inputs for property tests

* Fix advancing when passing over escaped dollar signs

* Switch to inline snapshot captures

* Cleanup invalid capture error formatting code

10 files changed, 901 insertions, 82 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 09911ab..8aeef89 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -12,6 +12,16 @@ dependencies = [
 ]
 
 [[package]]
+name = "ansi-to-html"
+version = "0.1.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c7bd918cc0ff933f0e6cf48a8f74584818ea43e07d1fba1f9251bb3df2a37ca2"
+dependencies = [
+ "regex",
+ "thiserror",
+]
+
+[[package]]
 name = "ansi_term"
 version = "0.12.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -55,7 +65,7 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5ca11d4be1bab0c8bc8734a9aa7bf4ee8316d462a08c6ac5052f888fef5b494b"
 dependencies = [
- "windows-sys",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -65,7 +75,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "f0699d10d2f4d628a98ee7b57b289abbc98ff3bad977cb3152709d4bf2330628"
 dependencies = [
  "anstyle",
- "windows-sys",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -96,6 +106,21 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d468802bab17cbc0cc575e9b053f41e72aa36bfa6b7f55e3529ffa43161b97fa"
 
 [[package]]
+name = "bit-set"
+version = "0.5.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0700ddab506f33b20a03b13996eccd309a48e5ff77d0d95926aa0210fb4e95f1"
+dependencies = [
+ "bit-vec",
+]
+
+[[package]]
+name = "bit-vec"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "349f9b6a179ed607305526ca489b34ad0a41aed5f7980fa90eb03160b69598fb"
+
+[[package]]
 name = "bitflags"
 version = "1.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -191,6 +216,19 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "acbf1af155f9b9ef647e42cdc158db4b64a1b61f743629225fde6f3e0be2a7c7"
 
 [[package]]
+name = "console"
+version = "0.15.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c926e00cc70edefdc64d3a5ff31cc65bb97a3460097762bd23afb4d8145fccf8"
+dependencies = [
+ "encode_unicode",
+ "lazy_static",
+ "libc",
+ "unicode-width",
+ "windows-sys 0.45.0",
+]
+
+[[package]]
 name = "crossbeam-deque"
 version = "0.8.3"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -242,13 +280,19 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a26ae43d7bcc3b814de94796a5e736d4029efb0ee900c12e2d54c993ad1a1e07"
 
 [[package]]
+name = "encode_unicode"
+version = "0.3.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a357d28ed41a50f9c765dbfe56cbc04a64e53e5fc58ba79fbc34c10ef3df831f"
+
+[[package]]
 name = "errno"
 version = "0.3.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ac3e13f66a2f95e32a39eaa81f6b95d42878ca0e1db0c7543723dfe12557e860"
 dependencies = [
  "libc",
- "windows-sys",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -264,6 +308,17 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3f9eec918d3f24069decb9af1554cad7c880e2da24a9afd88aca000531ab82c1"
 
 [[package]]
+name = "getrandom"
+version = "0.2.10"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "be4136b2a15dd319360be1c07d9933517ccf0be8f16bf62a3bee4f0d618df427"
+dependencies = [
+ "cfg-if",
+ "libc",
+ "wasi",
+]
+
+[[package]]
 name = "globset"
 version = "0.4.13"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -317,6 +372,19 @@ dependencies = [
 ]
 
 [[package]]
+name = "insta"
+version = "1.34.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5d64600be34b2fcfc267740a243fa7744441bb4947a619ac4e5bb6507f35fbfc"
+dependencies = [
+ "console",
+ "lazy_static",
+ "linked-hash-map",
+ "similar",
+ "yaml-rust",
+]
+
+[[package]]
 name = "is-terminal"
 version = "0.4.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -324,7 +392,7 @@ checksum = "cb0889898416213fab133e1d33a0e5858a48177452750691bde3666d0fdbaf8b"
 dependencies = [
  "hermit-abi",
  "rustix",
- "windows-sys",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -349,6 +417,18 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a08173bc88b7955d1b3145aa561539096c421ac8debde8cbc3612ec635fee29b"
 
 [[package]]
+name = "libm"
+version = "0.2.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4ec2a862134d2a7d32d7983ddcdd1c4923530833c9f2ea1a44fc5fa473989058"
+
+[[package]]
+name = "linked-hash-map"
+version = "0.5.6"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0717cef1bc8b636c6e1c1bbdefc09e6322da8a9321966e8928ef80d20f7f770f"
+
+[[package]]
 name = "linux-raw-sys"
 version = "0.4.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -385,12 +465,28 @@ dependencies = [
 ]
 
 [[package]]
+name = "num-traits"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "39e3200413f237f41ab11ad6d161bc7239c84dcb631773ccd7de3dfe4b5c267c"
+dependencies = [
+ "autocfg",
+ "libm",
+]
+
+[[package]]
 name = "once_cell"
 version = "1.18.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dd8b5dd2ae5ed71462c540258bedcb51965123ad7e7ccf4b9a8cafaa4a63576d"
 
 [[package]]
+name = "ppv-lite86"
+version = "0.2.17"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5b40af805b3121feab8a3c29f04d8ad262fa8e0561883e7653e024ae4479e6de"
+
+[[package]]
 name = "predicates"
 version = "3.0.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -428,6 +524,32 @@ dependencies = [
 ]
 
 [[package]]
+name = "proptest"
+version = "1.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7c003ac8c77cb07bb74f5f198bce836a689bcd5a42574612bf14d17bfd08c20e"
+dependencies = [
+ "bit-set",
+ "bit-vec",
+ "bitflags 2.4.1",
+ "lazy_static",
+ "num-traits",
+ "rand",
+ "rand_chacha",
+ "rand_xorshift",
+ "regex-syntax 0.7.5",
+ "rusty-fork",
+ "tempfile",
+ "unarray",
+]
+
+[[package]]
+name = "quick-error"
+version = "1.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a1d01941d82fa2ab50be1e79e6714289dd7cde78eba4c074bc5a4374f650dfe0"
+
+[[package]]
 name = "quote"
 version = "1.0.33"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -437,6 +559,45 @@ dependencies = [
 ]
 
 [[package]]
+name = "rand"
+version = "0.8.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "34af8d1a0e25924bc5b7c43c079c942339d8f0a8b57c39049bef581b46327404"
+dependencies = [
+ "libc",
+ "rand_chacha",
+ "rand_core",
+]
+
+[[package]]
+name = "rand_chacha"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6c10a63a0fa32252be49d21e7709d4d4baf8d231c2dbce1eaa8141b9b127d88"
+dependencies = [
+ "ppv-lite86",
+ "rand_core",
+]
+
+[[package]]
+name = "rand_core"
+version = "0.6.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
+dependencies = [
+ "getrandom",
+]
+
+[[package]]
+name = "rand_xorshift"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d25bf25ec5ae4a3f1b92f929810509a2f53d7dca2f50b794ff57e3face536c8f"
+dependencies = [
+ "rand_core",
+]
+
+[[package]]
 name = "rayon"
 version = "1.8.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -474,7 +635,7 @@ dependencies = [
  "aho-corasick",
  "memchr",
  "regex-automata",
- "regex-syntax",
+ "regex-syntax 0.8.2",
 ]
 
 [[package]]
@@ -485,11 +646,17 @@ checksum = "5f804c7828047e88b2d32e2d7fe5a105da8ee3264f01902f796c8e067dc2483f"
 dependencies = [
  "aho-corasick",
  "memchr",
- "regex-syntax",
+ "regex-syntax 0.8.2",
 ]
 
 [[package]]
 name = "regex-syntax"
+version = "0.7.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "dbb5fb1acd8a1a18b3dd5be62d25485eb770e05afb408a9627d14d451bae12da"
+
+[[package]]
+name = "regex-syntax"
 version = "0.8.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "c08c74e62047bb2de4ff487b251e4a92e24f48745648451635cec7d591162d9f"
@@ -510,7 +677,19 @@ dependencies = [
  "errno",
  "libc",
  "linux-raw-sys",
- "windows-sys",
+ "windows-sys 0.48.0",
+]
+
+[[package]]
+name = "rusty-fork"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cb3dcc6e454c328bb824492db107ab7c0ae8fcffe4ad210136ef014458c1bc4f"
+dependencies = [
+ "fnv",
+ "quick-error",
+ "tempfile",
+ "wait-timeout",
 ]
 
 [[package]]
@@ -532,17 +711,22 @@ checksum = "94143f37725109f92c262ed2cf5e59bce7498c01bcc1502d7b9afe439a4e9f49"
 name = "sd"
 version = "0.7.6"
 dependencies = [
+ "ansi-to-html",
  "ansi_term",
  "anyhow",
  "assert_cmd",
  "clap",
  "clap_mangen",
+ "console",
  "globwalk",
  "ignore",
+ "insta",
  "is-terminal",
  "memmap2",
+ "proptest",
  "rayon",
  "regex",
+ "regex-automata",
  "tempfile",
  "thiserror",
  "unescape",
@@ -569,6 +753,12 @@ dependencies = [
 ]
 
 [[package]]
+name = "similar"
+version = "2.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2aeaf503862c419d66959f5d7ca015337d864e9c49485d771b732e2a20453597"
+
+[[package]]
 name = "strsim"
 version = "0.10.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -595,7 +785,7 @@ dependencies = [
  "fastrand",
  "redox_syscall",
  "rustix",
- "windows-sys",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -605,7 +795,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "21bebf2b7c9e0a515f6e0f8c51dc0f8e4696391e6f1ff30379559f8365fb0df7"
 dependencies = [
  "rustix",
- "windows-sys",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]
@@ -645,6 +835,12 @@ dependencies = [
 ]
 
 [[package]]
+name = "unarray"
+version = "0.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eaea85b334db583fe3274d12b4cd1880032beab409c0d774be044d4480ab9a94"
+
+[[package]]
 name = "unescape"
 version = "0.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -657,6 +853,12 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "3354b9ac3fae1ff6755cb6db53683adb661634f67557942dea4facebec0fee4b"
 
 [[package]]
+name = "unicode-width"
+version = "0.1.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e51733f11c9c4f72aa0c160008246859e340b00807569a0da0e7a1079b27ba85"
+
+[[package]]
 name = "utf8parse"
 version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -682,6 +884,12 @@ dependencies = [
 ]
 
 [[package]]
+name = "wasi"
+version = "0.11.0+wasi-snapshot-preview1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9c8d87e72b64a3b4db28d11ce29237c246188f4f51057d65a7eab63b7987e423"
+
+[[package]]
 name = "winapi"
 version = "0.3.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -714,11 +922,35 @@ checksum = "712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f"
 
 [[package]]
 name = "windows-sys"
+version = "0.45.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "75283be5efb2831d37ea142365f009c02ec203cd29a3ebecbc093d52315b66d0"
+dependencies = [
+ "windows-targets 0.42.2",
+]
+
+[[package]]
+name = "windows-sys"
 version = "0.48.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "677d2418bec65e3338edb076e806bc1ec15693c5d0104683f2efe857f61056a9"
 dependencies = [
- "windows-targets",
+ "windows-targets 0.48.5",
+]
+
+[[package]]
+name = "windows-targets"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8e5180c00cd44c9b1c88adb3693291f1cd93605ded80c250a75d472756b4d071"
+dependencies = [
+ "windows_aarch64_gnullvm 0.42.2",
+ "windows_aarch64_msvc 0.42.2",
+ "windows_i686_gnu 0.42.2",
+ "windows_i686_msvc 0.42.2",
+ "windows_x86_64_gnu 0.42.2",
+ "windows_x86_64_gnullvm 0.42.2",
+ "windows_x86_64_msvc 0.42.2",
 ]
 
 [[package]]
@@ -727,53 +959,95 @@ version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "9a2fa6e2155d7247be68c096456083145c183cbbbc2764150dda45a87197940c"
 dependencies = [
- "windows_aarch64_gnullvm",
- "windows_aarch64_msvc",
- "windows_i686_gnu",
- "windows_i686_msvc",
- "windows_x86_64_gnu",
- "windows_x86_64_gnullvm",
- "windows_x86_64_msvc",
+ "windows_aarch64_gnullvm 0.48.5",
+ "windows_aarch64_msvc 0.48.5",
+ "windows_i686_gnu 0.48.5",
+ "windows_i686_msvc 0.48.5",
+ "windows_x86_64_gnu 0.48.5",
+ "windows_x86_64_gnullvm 0.48.5",
+ "windows_x86_64_msvc 0.48.5",
 ]
 
 [[package]]
 name = "windows_aarch64_gnullvm"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "597a5118570b68bc08d8d59125332c54f1ba9d9adeedeef5b99b02ba2b0698f8"
+
+[[package]]
+name = "windows_aarch64_gnullvm"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "2b38e32f0abccf9987a4e3079dfb67dcd799fb61361e53e2882c3cbaf0d905d8"
 
 [[package]]
 name = "windows_aarch64_msvc"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e08e8864a60f06ef0d0ff4ba04124db8b0fb3be5776a5cd47641e942e58c4d43"
+
+[[package]]
+name = "windows_aarch64_msvc"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "dc35310971f3b2dbbf3f0690a219f40e2d9afcf64f9ab7cc1be722937c26b4bc"
 
 [[package]]
 name = "windows_i686_gnu"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c61d927d8da41da96a81f029489353e68739737d3beca43145c8afec9a31a84f"
+
+[[package]]
+name = "windows_i686_gnu"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "a75915e7def60c94dcef72200b9a8e58e5091744960da64ec734a6c6e9b3743e"
 
 [[package]]
 name = "windows_i686_msvc"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "44d840b6ec649f480a41c8d80f9c65108b92d89345dd94027bfe06ac444d1060"
+
+[[package]]
+name = "windows_i686_msvc"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "8f55c233f70c4b27f66c523580f78f1004e8b5a8b659e05a4eb49d4166cca406"
 
 [[package]]
 name = "windows_x86_64_gnu"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8de912b8b8feb55c064867cf047dda097f92d51efad5b491dfb98f6bbb70cb36"
+
+[[package]]
+name = "windows_x86_64_gnu"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "53d40abd2583d23e4718fddf1ebec84dbff8381c07cae67ff7768bbf19c6718e"
 
 [[package]]
 name = "windows_x86_64_gnullvm"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "26d41b46a36d453748aedef1486d5c7a85db22e56aff34643984ea85514e94a3"
+
+[[package]]
+name = "windows_x86_64_gnullvm"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b7b52767868a23d5bab768e390dc5f5c55825b6d30b86c844ff2dc7414044cc"
 
 [[package]]
 name = "windows_x86_64_msvc"
+version = "0.42.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9aec5da331524158c6d1a4ac0ab1541149c0b9505fde06423b02f5ef0106b9f0"
+
+[[package]]
+name = "windows_x86_64_msvc"
 version = "0.48.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ed94fce61571a4006852b7389a063ab983c02eb1bb37b47f8272ce92d06d9538"
@@ -787,3 +1061,12 @@ dependencies = [
  "clap_mangen",
  "roff",
 ]
+
+[[package]]
+name = "yaml-rust"
+version = "0.4.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "56c1936c4cc7a1c9ab21a1ebb602eb942ba868cbd44a99cb7cdc5892335e1c85"
+dependencies = [
+ "linked-hash-map",
+]
diff --git a/Cargo.toml b/Cargo.toml
index 3216193..1d9ab60 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -43,6 +43,11 @@ clap.workspace = true
 assert_cmd = "2.0.12"
 anyhow = "1.0.75"
 clap_mangen = "0.2.14"
+proptest = "1.3.1"
+console = "0.15.7"
+insta = "1.34.0"
+ansi-to-html = "0.1.3"
+regex-automata = "0.4.3"
 
 [profile.release]
 opt-level = 3
diff --git a/proptest-regressions/replacer/tests.txt b/proptest-regressions/replacer/tests.txt
new file mode 100644
index 0000000..f164b58
--- /dev/null
+++ b/proptest-regressions/replacer/tests.txt
@@ -0,0 +1,8 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc 3a23ade8355ca034558ea8635e4ea2ee96ecb38b7b1cb9a854509d7633d45795 # shrinks to s = ""
+cc 8c8d1e7497465f26416bddb7607df0de1fce48d098653eeabac0ad2aeba1fa0a # shrinks to s = "$0$0a"
diff --git a/proptest-regressions/replacer/validate.txt b/proptest-regressions/replacer/validate.txt
new file mode 100644
index 0000000..bbdb3b3
--- /dev/null
+++ b/proptest-regressions/replacer/validate.txt
@@ -0,0 +1,10 @@
+# Seeds for failure cases proptest has generated in the past. It is
+# automatically read and these particular cases re-run before any
+# novel cases are generated.
+#
+# It is recommended to check this file in to source control so that
+# everyone who runs the test benefits from these saved cases.
+cc cfacd65058c8dae0ac7b91c56b8096c36ef68cb35d67262debebac005ea9c677 # shrinks to s = ""
+cc 61e5dc6ce0314cde48b5cbc839fbf46a49fcf8d0ba02cfeecdcbff52fca8c786 # shrinks to s = "$a"
+cc 8e5fd9dbb58ae762a751349749320664715056ef63aad58215397e87ee42c722 # shrinks to s = "$$"
+cc 37c2e41ceeddbecbc4e574f82b58a4007923027ad1a6756bf2f547aa3f748d13 # shrinks to s = "$$0"
diff --git a/src/error.rs b/src/error.rs
index e757cf4..517defd 100644
--- a/src/error.rs
+++ b/src/error.rs
@@ -3,6 +3,8 @@ use std::{
     path::PathBuf,
 };
 
+use crate::replacer::InvalidReplaceCapture;
+
 #[derive(thiserror::Error)]
 pub enum Error {
     #[error("invalid regex {0}")]
@@ -15,6 +17,8 @@ pub enum Error {
     InvalidPath(PathBuf),
     #[error("failed processing files:\n{0}")]
     FailedProcessing(FailedJobs),
+    #[error("{0}")]
+    InvalidReplaceCapture(#[from] InvalidReplaceCapture),
 }
 
 pub struct FailedJobs(Vec<(PathBuf, Error)>);
diff --git a/src/main.rs b/src/main.rs
index de24644..c7097b4 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,13 +5,23 @@ mod input;
 pub(crate) mod replacer;
 pub(crate) mod utils;
 
+use std::process;
+
 pub(crate) use self::input::{App, Source};
+use ansi_term::{Color, Style};
 pub(crate) use error::{Error, Result};
 use replacer::Replacer;
 
 use clap::Parser;
 
-fn main() -> Result<()> {
+fn main() {
+    if let Err(e) = try_main() {
+        eprintln!("{}: {}", Style::from(Color::Red).bold().paint("error"), e);
+        process::exit(1);
+    }
+}
+
+fn try_main() -> Result<()> {
     let options = cli::Options::parse();
 
     let source = if options.recursive {
diff --git a/src/replacer.rs b/src/replacer/mod.rs
index f6d5d21..b6e0c0b 100644
--- a/src/replacer.rs
+++ b/src/replacer/mod.rs
@@ -1,6 +1,14 @@
+use std::{fs, fs::File, io::prelude::*, path::Path};
+
 use crate::{utils, Error, Result};
+
 use regex::bytes::Regex;
-use std::{fs, fs::File, io::prelude::*, path::Path};
+
+#[cfg(test)]
+mod tests;
+mod validate;
+
+pub use validate::{validate_replace, InvalidReplaceCapture};
 
 pub(crate) struct Replacer {
     regex: Regex,
@@ -20,6 +28,8 @@ impl Replacer {
         let (look_for, replace_with) = if is_literal {
             (regex::escape(&look_for), replace_with.into_bytes())
         } else {
+            validate_replace(&replace_with)?;
+
             (
                 look_for,
                 utils::unescape(&replace_with)
@@ -154,66 +164,3 @@ impl Replacer {
         Ok(())
     }
 }
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    fn replace(
-        look_for: impl Into<String>,
-        replace_with: impl Into<String>,
-        literal: bool,
-        flags: Option<&'static str>,
-        src: &'static str,
-        target: &'static str,
-    ) {
-        let replacer = Replacer::new(
-            look_for.into(),
-            replace_with.into(),
-            literal,
-            flags.map(ToOwned::to_owned),
-            None,
-        )
-        .unwrap();
-        assert_eq!(
-            std::str::from_utf8(&replacer.replace(src.as_bytes())),
-            Ok(target)
-        );
-    }
-
-    #[test]
-    fn default_global() {
-        replace("a", "b", false, None, "aaa", "bbb");
-    }
-
-    #[test]
-    fn escaped_char_preservation() {
-        replace("a", "b", false, None, "a\\n", "b\\n");
-    }
-
-    #[test]
-    fn case_sensitive_default() {
-        replace("abc", "x", false, None, "abcABC", "xABC");
-        replace("abc", "x", true, None, "abcABC", "xABC");
-    }
-
-    #[test]
-    fn sanity_check_literal_replacements() {
-        replace("((special[]))", "x", true, None, "((special[]))y", "xy");
-    }
-
-    #[test]
-    fn unescape_regex_replacements() {
-        replace("test", r"\n", false, None, "testtest", "\n\n");
-    }
-
-    #[test]
-    fn no_unescape_literal_replacements() {
-        replace("test", r"\n", true, None, "testtest", r"\n\n");
-    }
-
-    #[test]
-    fn full_word_replace() {
-        replace("abc", "def", false, Some("w"), "abcd abc", "abcd def");
-    }
-}
diff --git a/src/replacer/tests.rs b/src/replacer/tests.rs
new file mode 100644
index 0000000..9304a17
--- /dev/null
+++ b/src/replacer/tests.rs
@@ -0,0 +1,80 @@
+use super::*;
+
+use proptest::prelude::*;
+
+proptest! {
+    #[test]
+    fn validate_doesnt_panic(s in r"(\PC*\$?){0,5}") {
+        let _ = validate::validate_replace(&s);
+    }
+
+    // $ followed by a digit and a non-ident char or an ident char
+    #[test]
+    fn validate_ok(s in r"([^\$]*(\$([0-9][^a-zA-Z_0-9\$]|a-zA-Z_))?){0,5}") {
+        validate::validate_replace(&s).unwrap();
+    }
+
+    // Force at least one $ followed by a digit and an ident char
+    #[test]
+    fn validate_err(s in r"[^\$]*?\$[0-9][a-zA-Z_]\PC*") {
+        validate::validate_replace(&s).unwrap_err();
+    }
+}
+
+fn replace(
+    look_for: impl Into<String>,
+    replace_with: impl Into<String>,
+    literal: bool,
+    flags: Option<&'static str>,
+    src: &'static str,
+    target: &'static str,
+) {
+    let replacer = Replacer::new(
+        look_for.into(),
+        replace_with.into(),
+        literal,
+        flags.map(ToOwned::to_owned),
+        None,
+    )
+    .unwrap();
+    assert_eq!(
+        std::str::from_utf8(&replacer.replace(src.as_bytes())),
+        Ok(target)
+    );
+}
+
+#[test]
+fn default_global() {
+    replace("a", "b", false, None, "aaa", "bbb");
+}
+
+#[test]
+fn escaped_char_preservation() {
+    replace("a", "b", false, None, "a\\n", "b\\n");
+}
+
+#[test]
+fn case_sensitive_default() {
+    replace("abc", "x", false, None, "abcABC", "xABC");
+    replace("abc", "x", true, None, "abcABC", "xABC");
+}
+
+#[test]
+fn sanity_check_literal_replacements() {
+    replace("((special[]))", "x", true, None, "((special[]))y", "xy");
+}
+
+#[test]
+fn unescape_regex_replacements() {
+    replace("test", r"\n", false, None, "testtest", "\n\n");
+}
+
+#[test]
+fn no_unescape_literal_replacements() {
+    replace("test", r"\n", true, None, "testtest", r"\n\n");
+}
+
+#[test]
+fn full_word_replace() {
+    replace("abc", "def", false, Some("w"), "abcd abc", "abcd def");
+}
diff --git a/src/replacer/validate.rs b/src/replacer/validate.rs
new file mode 100644
index 0000000..da5cc71
--- /dev/null
+++ b/src/replacer/validate.rs
@@ -0,0 +1,380 @@
+use std::{error::Error, fmt, str::CharIndices};
+
+use ansi_term::{Color, Style};
+
+#[derive(Debug)]
+pub struct InvalidReplaceCapture {
+    original_replace: String,
+    invalid_ident: Span,
+    num_leading_digits: usize,
+}
+
+impl Error for InvalidReplaceCapture {}
+
+// NOTE: This code is much more allocation heavy than it needs to be, but it's
+//       only displayed as a hard error to the user, so it's not a big deal
+impl fmt::Display for InvalidReplaceCapture {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        #[derive(Clone, Copy)]
+        enum SpecialChar {
+            Newline,
+            CarriageReturn,
+            Tab,
+        }
+
+        impl SpecialChar {
+            fn new(c: char) -> Option<Self> {
+                match c {
+                    '\n' => Some(Self::Newline),
+                    '\r' => Some(Self::CarriageReturn),
+                    '\t' => Some(Self::Tab),
+                    _ => None,
+                }
+            }
+
+            /// Renders as the character from the "Control Pictures" block
+            ///
+            /// https://en.wikipedia.org/wiki/Control_Pictures
+            fn render(self) -> char {
+                match self {
+                    Self::Newline => '␊',
+                    Self::CarriageReturn => '␍',
+                    Self::Tab => '␉',
+                }
+            }
+        }
+
+        let Self {
+            original_replace,
+            invalid_ident,
+            num_leading_digits,
+        } = self;
+
+        // Build up the error to show the user
+        let mut formatted = String::new();
+        let mut arrows_start = Span::start_at(0);
+        let special = Style::new().bold();
+        let error = Style::from(Color::Red).bold();
+        for (byte_index, c) in original_replace.char_indices() {
+            let (prefix, suffix, text) = match SpecialChar::new(c) {
+                Some(c) => {
+                    (Some(special.prefix()), Some(special.suffix()), c.render())
+                }
+                None => {
+                    let (prefix, suffix) = if byte_index == invalid_ident.start
+                    {
+                        (Some(error.prefix()), None)
+                    } else if byte_index
+                        == invalid_ident.end.checked_sub(1).unwrap()
+                    {
+                        (None, Some(error.suffix()))
+                    } else {
+                        (None, None)
+                    };
+                    (prefix, suffix, c)
+                }
+            };
+
+            if let Some(prefix) = prefix {
+                formatted.push_str(&prefix.to_string());
+            }
+            formatted.push(text);
+            if let Some(suffix) = suffix {
+                formatted.push_str(&suffix.to_string());
+            }
+
+            if byte_index < invalid_ident.start {
+                // Assumes that characters have a base display width of 1. While
+                // that's not technically true, it's near impossible to do right
+                // since the specifics on text rendering is up to the user's
+                // terminal/font. This _does_ rely on variable-width characters
+                // like \n, \r, and \t getting converting to single character
+                // representations above
+                arrows_start.start += 1;
+            }
+        }
+
+        // This relies on all non-curly-braced capture chars being 1 byte
+        let arrows_span = arrows_start.end_offset(invalid_ident.len());
+        let mut arrows = " ".repeat(arrows_span.start);
+        arrows.push_str(&format!(
+            "{}",
+            Style::new().bold().paint("^".repeat(arrows_span.len()))
+        ));
+
+        let ident = invalid_ident.slice(original_replace);
+        let (number, the_rest) = ident.split_at(*num_leading_digits);
+        let disambiguous = format!("${{{number}}}{the_rest}");
+        let error_message = format!(
+            "The numbered capture group `{}` in the replacement text is ambiguous.",
+            Style::new().bold().paint(format!("${}", number).to_string())
+        );
+        let hint_message = format!(
+            "{}: Use curly braces to disambiguate it `{}`.",
+            Style::from(Color::Blue).bold().paint("hint"),
+            Style::new().bold().paint(disambiguous)
+        );
+
+        writeln!(f, "{}", error_message)?;
+        writeln!(f, "{}", hint_message)?;
+        writeln!(f, "{}", formatted)?;
+        write!(f, "{}", arrows)
+    }
+}
+
+pub fn validate_replace(s: &str) -> Result<(), InvalidReplaceCapture> {
+    for ident in ReplaceCaptureIter::new(s) {
+        let mut char_it = ident.name.char_indices();
+        let (_, c) = char_it.next().unwrap();
+        if c.is_ascii_digit() {
+            for (i, c) in char_it {
+                if !c.is_ascii_digit() {
+                    return Err(InvalidReplaceCapture {
+                        original_replace: s.to_owned(),
+                        invalid_ident: ident.span,
+                        num_leading_digits: i,
+                    });
+                }
+            }
+        }
+    }
+
+    Ok(())
+}
+
+#[derive(Clone, Copy, Debug)]
+struct Span {
+    start: usize,
+    end: usize,
+}
+
+impl Span {
+    fn start_at(start: usize) -> SpanOpen {
+        SpanOpen { start }
+    }
+
+    fn new(start: usize, end: usize) -> Self {
+        // `<` instead of `<=` because `Span` is exclusive on the upper bound
+        assert!(start < end);
+        Self { start, end }
+    }
+
+    fn slice(self, s: &str) -> &str {
+        &s[self.start..self.end]
+    }
+
+    fn len(self) -> usize {
+        self.end - self.start
+    }
+}
+
+#[derive(Clone, Copy)]
+struct SpanOpen {
+    start: usize,
+}
+
+impl SpanOpen {
+    fn end_at(self, end: usize) -> Span {
+        let Self { start } = self;
+        Span::new(start, end)
+    }
+
+    fn end_offset(self, offset: usize) -> Span {
+        assert_ne!(offset, 0);
+        let Self { start } = self;
+        self.end_at(start + offset)
+    }
+}
+
+#[derive(Debug)]
+struct Capture<'rep> {
+    name: &'rep str,
+    span: Span,
+}
+
+impl<'rep> Capture<'rep> {
+    fn new(name: &'rep str, span: Span) -> Self {
+        Self { name, span }
+    }
+}
+
+/// An iterator over the capture idents in an interpolated replacement string
+///
+/// This code is adapted from the `regex` crate
+/// <https://docs.rs/regex-automata/latest/src/regex_automata/util/interpolate.rs.html>
+/// (hence the high quality doc comments).
+struct ReplaceCaptureIter<'rep>(CharIndices<'rep>);
+
+impl<'rep> ReplaceCaptureIter<'rep> {
+    fn new(s: &'rep str) -> Self {
+        Self(s.char_indices())
+    }
+}
+
+impl<'rep> Iterator for ReplaceCaptureIter<'rep> {
+    type Item = Capture<'rep>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // Continually seek to `$` until we find one that has a capture group
+