bash/zsh: add notes for regex matching

2 files changed, 37 insertions, 0 deletions

diff --git a/src/tools/bash.md b/src/tools/bash.md
index 201a6ad..15f455a 100644
--- a/src/tools/bash.md
+++ b/src/tools/bash.md
@@ -136,6 +136,25 @@ parse_args -f xxx -c
 parse_args -f yyy
 ```
 
+## Regular Expressions
+
+Bash supports regular expression matching with the binary operator `=~`.
+The match results can be accessed via the `$BASH_REMATCH` variable:
+- `${BASH_REMATCH[0]}` contains the full match
+- `${BASH_REMATCH[1]}` contains match of the first capture group
+
+```bash
+INPUT='title foo : 1234'
+REGEX='^title (.+) : ([0-9]+)$'
+if [[ $INPUT =~ $REGEX ]]; then
+    echo "${BASH_REMATCH[0]}"    # title foo : 1234
+    echo "${BASH_REMATCH[1]}"    # foo
+    echo "${BASH_REMATCH[2]}"    # 1234
+fi
+```
+> **Caution**: When specifying a `regex` in the `[[ ]]` block directly, quotes will be treated as part of the pattern.
+> `[[ $INPUT =~ "foo" ]]` will match against `"foo"` not `foo`!
+
 ## Completion
 
 The `complete` builtin is used to interact with the completion system.
diff --git a/src/tools/zsh.md b/src/tools/zsh.md
index 4d52ae0..e9aa8bf 100644
--- a/src/tools/zsh.md
+++ b/src/tools/zsh.md
@@ -162,6 +162,24 @@ echo ${(L)foo}        # aabb
 echo ${(U)foo}        # AABB
 ```
 
+## Regular Expressions
+
+Zsh supports regular expression matching with the binary operator `=~`.
+The match results can be accessed via the `$MATCH` variable and
+`$match` indexed array:
+- `$MATCH` contains the full match
+- `$match[1]` contains match of the first capture group
+
+```zsh
+INPUT='title foo : 1234'
+REGEX='^title (.+) : ([0-9]+)$'
+if [[ $INPUT =~ $REGEX ]]; then
+    echo "$MATCH"       # title foo : 1234
+    echo "$match[1]"    # foo
+    echo "$match[2]"    # 1234
+fi
+```
+
 ## Completion
 
 ### Installation