[PATCH 1/1] Added built in function recognition for shell

[Moumita <dhar61595@gmail.com>]

Introduced a built-in userdiff driver for shell scripts, enabling
accurate function name recognition in `git diff` hunk headers.

Enhancements include:
- Function name detection for both POSIX and Bash/Ksh-style functions:
  - `function_name() { ... }`
  - `function function_name { ... }`
- Exclusion of shell keywords that can resemble function names,
  preventing false matches (e.g., `if`, `for`, `while`, `return`, etc.).
- Improved tokenization support for:
  - Identifiers (variable and function names)
  - Numeric constants (integers and decimals)
  - Shell variables (`$VAR`, `${VAR}`)
  - Logical (`&&`, `||`, `==`, `!=`, `<=`, `>=`) and arithmetic operators
  - Assignment and redirection operators
  - Brackets and grouping symbols

This update improves Git’s diff readability for shell scripts,
bringing it in line with existing built-in userdiff drivers.

Signed-off-by: Moumita <dhar61595@gmail.com>
---
 userdiff.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/userdiff.c b/userdiff.c
index 340c4eb4f7..a8c14807c6 100644
--- a/userdiff.c
+++ b/userdiff.c
@@ -334,6 +334,26 @@ PATTERNS("scheme",
 	 "\\|([^\\\\]*)\\|"
 	 /* All other words should be delimited by spaces or parentheses */
 	 "|([^][)(}{[ \t])+"),
+PATTERNS("shell",
+	 /* Negate shell keywords that can look like functions */
+	 "!^[ \t]*(if|elif|else|fi|for|while|until|case|esac|then|do|done|return|break|continue)\\b\n"
+	 /* POSIX-style shell functions: function_name() { ... } */
+	 "^[ \t]*([a-zA-Z_][a-zA-Z0-9_]*)[ \t]*\\(\\)[ \t]*\\{\n"
+	 /* Bash/Ksh-style functions: function function_name { ... } */
+	 "^[ \t]*function[ \t]+([a-zA-Z_][a-zA-Z0-9_]*)[ \t]*\\{\n",
+	 /* -- */
+	 /* Identifiers: variable and function names */
+	 "[a-zA-Z_][a-zA-Z0-9_]*"
+	 /* Numeric constants: integers and decimals */
+	 "|[-+]?[0-9]+(\\.[0-9]*)?"
+	 /* Shell variables: $VAR and ${VAR} */
+	 "|\\$[a-zA-Z_][a-zA-Z0-9_]*|\\$\\{[^}]+\\}"
+	 /* Logical and comparison operators */
+	 "|\\|\\||&&|<<|>>|==|!=|<=|>="
+	 /* Assignment and arithmetic operators */
+	 "|[-+*/%&|^!=<>]=?"
+	 /* Brackets and grouping symbols */
+	 "|\\(|\\)|\\{|\\}|\\[|\\]"),
 PATTERNS("tex", "^(\\\\((sub)*section|chapter|part)\\*{0,1}\\{.*)$",
 	 "\\\\[a-zA-Z@]+|\\\\.|([a-zA-Z0-9]|[^\x01-\x7f])+"),
 { .name = "default", .binary = -1 },
-- 
2.48.0