1	#!/bin/bash
2	#
3	# Only bash and zsh seem to implement [[ foo =~ '' ]]
4	#
5	# ^(a b)$ is a regex that should match 'a b' in a group.
6	#
7	# Not sure what bash is doing here... I think I have to just be empirical.
8	# Might need "compat" switch for parsing the regex. It should be an opaque
9	# string like zsh, not sure why it isn't.
10	#
11	# I think this is just papering over bugs...
12	# https://www.gnu.org/software/bash/manual/bash.html#Conditional-Constructs
13	#
14	# Storing the regular expression in a shell variable is often a useful way to
15	# avoid problems with quoting characters that are special to the shell. It is
16	# sometimes difficult to specify a regular expression literally without using
17	# quotes, or to keep track of the quoting used by regular expressions while
18	# paying attention to the shell’s quote removal. Using a shell variable to
19	# store the pattern decreases these problems. For example, the following is
20	# equivalent to the above:
21	#
22	# pattern='[[:space:]]*(a)?b'
23	# [[ $line =~ $pattern ]]
24	#
25	# If you want to match a character that’s special to the regular expression
26	# grammar, it has to be quoted to remove its special meaning. This means that in
27	# the pattern ‘xxx.txt’, the ‘.’ matches any character in the string (its usual
28	# regular expression meaning), but in the pattern ‘"xxx.txt"’ it can only match a
29	# literal ‘.’. Shell programmers should take special care with backslashes, since
30	# backslashes are used both by the shell and regular expressions to remove the
31	# special meaning from the following character. The following two sets of
32	# commands are not equivalent:
33	#
34	# From bash code: ( | ) are treated special. Normally they must be quoted, but
35	# they can be UNQUOTED in BASH_REGEX state. In fact they can't be quoted!
36
37	### Match is unanchored at both ends
38	[[ 'bar' =~ a ]] && echo true
39	# stdout: true
40
41	### Failed match
42	[[ 'bar' =~ X ]] && echo true
43	# stdout-json: ""
44
45	### Regex quoted with \ -- preferred in bash
46	[[ 'a b' =~ ^(a\ b)$ ]] && echo true
47	# stdout: true
48
49	### Regex quoted with single quotes
50	# bash doesn't like the quotes
51	[[ 'a b' =~ '^(a b)$' ]] && echo true
52	# stdout: true
53	# OK bash stdout-json: ""
54
55	### Regex quoted with double quotes
56	# bash doesn't like the quotes
57	[[ 'a b' =~ "^(a b)$" ]] && echo true
58	# stdout: true
59	# OK bash stdout-json: ""
60
61	### Fix single quotes by storing in variable
62	pat='^(a b)$'
63	[[ 'a b' =~ $pat ]] && echo true
64	# stdout: true
65
66	### Fix single quotes by storing in variable
67	pat="^(a b)$"
68	[[ 'a b' =~ $pat ]] && echo true
69	# stdout: true
70
71	### Double quoting pat variable -- again bash doesn't like it.
72	pat="^(a b)$"
73	[[ 'a b' =~ "$pat" ]] && echo true
74	# stdout: true
75	# OK bash stdout-json: ""
76
77	### Regex with == and not =~ is parse error, different lexer mode required
78	# They both give a syntax error. This is lame.
79	[[ '^(a b)$' == ^(a\ b)$ ]] && echo true
80	# status: 2
81	# OK zsh status: 1
82
83	### Omitting ( )
84	[[ '^a b$' == ^a\ b$ ]] && echo true
85	# stdout: true
86
87	### Malformed regex
88	# Are they trying to PARSE the regex? Do they feed the buffer directly to
89	# regcomp()?
90	[[ 'a b' =~ ^)a\ b($ ]] && echo true
91	# status: 2
92	# OK zsh status: 1
93
94	### Regex with char class
95	# For some reason it doesn't work without parens?
96	[[ 'ba ba ' =~ ([a b]+) ]] && echo true
97	# stdout: true
98
99	### Operators lose meaning in () in regex state (BASH_REGEX_CAHRS)
100	[[ 'gt;' =~ (gt;) ]] && echo true
101	# stdout: true
102	# N-I zsh stdout-json: ""
103	# N-I zsh status: 1
104
105	### Regex with |
106	[[ 'bar' =~ foo|bar ]] && echo true
107	# stdout: true
108	# N-I zsh stdout-json: ""
109	# N-I zsh status: 1