[RFC 3/5] [gdb/testsuite] Update regexp in string_to_regexp

[Tom de Vries <tdevries@suse.de>]

There are two similar regexps, in string_to_regexp and with_gdb_prompt:
...
    regsub -all {[]?*+.|(){}^$\[\\]} $str {\\&} result
    regsub -all {[]*+.|()^$\[\\]} $prompt {\\&} prompt
...

In both cases, emacs tcl-mode has problems with it: auto indent doesn't
continue at column 4, but is reset to column 0.

The first regexp is basically one big bracket expression containing the chars:
...
  ] ? * + . | ( ) { } ^ $ \[ \\
...

There are a couple of ways that a ']' can be enclosed in a bracket
expression [1]:
- make it the first char
- escape using backslash
- make it a collating element

The first solution was chosen, but confuses tcl-mode.

Fix this by adding a backslash.

[1] https://www.tcl-lang.org/man/tcl8.6/TclCmd/re_syntax.htm#M30
---
 gdb/testsuite/lib/gdb-utils.exp | 4 +++-
 gdb/testsuite/lib/gdb.exp       | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/gdb/testsuite/lib/gdb-utils.exp b/gdb/testsuite/lib/gdb-utils.exp
index cabeb7aa1ed..874de79e6d8 100644
--- a/gdb/testsuite/lib/gdb-utils.exp
+++ b/gdb/testsuite/lib/gdb-utils.exp
@@ -36,7 +36,9 @@ proc gdb_init_commands {} {
 
 proc string_to_regexp {str} {
     set result $str
-    regsub -all {[]?*+.|(){}^$\[\\]} $str {\\&} result
+    # We use a backslash to escape the closing square bracket, even if it's
+    # the first character, to unconfuse emacs tcl-mode.
+    regsub -all {[\]?*+.|(){}^$\[\\]} $str {\\&} result
     return $result
 }
 
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index d4a471ff7c4..b0a4cc1b39f 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -3650,7 +3650,9 @@ proc with_gdb_prompt { prompt body } {
     # we start recording both forms separately instead of just $gdb_prompt.
     # The testsuite is pretty-much hardwired to interpret $gdb_prompt as the
     # regexp form.
-    regsub -all {[]*+.|()^$\[\\]} $prompt {\\&} prompt
+    # We use a backslash to escape the closing square bracket, even if it's
+    # the first character, to unconfuse emacs tcl-mode.
+    regsub -all {[\]*+.|()^$\[\\]} $prompt {\\&} prompt
 
     set saved $gdb_prompt
 
-- 
2.51.0