| 1 | ---
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| 2 | in_progress: yes
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| 3 | body_css_class: width40 help-body
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| 4 | default_highlighter: oils-sh
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| 5 | preserve_anchor_case: yes
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| 6 | ---
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| 7 |
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| 8 | YSH Expression Language
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| 9 | ===
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| 10 |
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| 11 | This chapter in the [Oils Reference](index.html) describes the YSH expression
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| 12 | language, which includes [Egg Expressions]($xref:eggex).
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| 13 |
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| 14 | <div id="toc">
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| 15 | </div>
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| 16 |
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| 17 | ## Literals
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| 18 |
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| 19 | ### bool-literal
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| 20 |
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| 21 | YSH uses JavaScript-like spellings for these three "atoms":
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| 22 |
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| 23 | true false null
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| 24 |
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| 25 | Note that the empty string is a good "special" value in some cases. The `null`
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| 26 | value can't be interpolated into words.
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| 27 |
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| 28 | ### int-literal
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| 29 |
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| 30 | var myint = 42
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| 31 | var myfloat = 3.14
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| 32 | var float2 = 1e100
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| 33 |
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| 34 | ### rune-literal
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| 35 |
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| 36 | #'a' #'_' \n \\ \u{3bc}
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| 37 |
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| 38 | ### ysh-string
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| 39 |
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| 40 | Double quoted strings are identical to shell:
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| 41 |
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| 42 | var dq = "hello $world and $(hostname)"
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| 43 |
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| 44 | Single quoted strings may be raw:
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| 45 |
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| 46 | var s = r'line\n' # raw string means \n is literal, NOT a newline
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| 47 |
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| 48 | Or escaped *J8 strings*:
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| 49 |
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| 50 | var s = u'line\n \u{3bc}' # unicode string means \n is a newline
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| 51 | var s = b'line\n \u{3bc} \yff' # same thing, but also allows bytes
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| 52 |
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| 53 | Both `u''` and `b''` strings evaluate to the single `Str` type. The difference
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| 54 | is that `b''` strings allow the `\yff` byte escape.
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| 55 |
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| 56 | ---
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| 57 |
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| 58 | There's no way to express a single quote in raw strings. Use one of the other
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| 59 | forms instead:
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| 60 |
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| 61 | var sq = "single quote: ' "
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| 62 | var sq = u'single quote: \' '
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| 63 |
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| 64 | Sometimes you can omit the `r`, e.g. where there are no backslashes and thus no
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| 65 | ambiguity:
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| 66 |
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| 67 | echo 'foo'
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| 68 | echo r'foo' # same thing
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| 69 |
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| 70 | The `u''` and `b''` strings are called *J8 strings* because the syntax in YSH
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| 71 | **code** matches JSON-like **data**.
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| 72 |
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| 73 | var strU = u'mu = \u{3bc}' # J8 string with escapes
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| 74 | var strB = b'bytes \yff' # J8 string that can express byte strings
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| 75 |
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| 76 | More examples:
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| 77 |
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| 78 | var myRaw = r'[a-z]\n' # raw strings are useful for regexes (not
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| 79 | # eggexes)
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| 80 |
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| 81 | ### triple-quoted
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| 82 |
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| 83 | Triple-quoted string literals have leading whitespace stripped on each line.
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| 84 | They come in the same variants:
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| 85 |
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| 86 | var dq = """
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| 87 | hello $world and $(hostname)
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| 88 | no leading whitespace
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| 89 | """
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| 90 |
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| 91 | var myRaw = r'''
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| 92 | raw string
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| 93 | no leading whitespace
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| 94 | '''
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| 95 |
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| 96 | var strU = u'''
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| 97 | string that happens to be unicode \u{3bc}
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| 98 | no leading whitespace
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| 99 | '''
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| 100 |
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| 101 | var strB = b'''
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| 102 | string that happens to be bytes \u{3bc} \yff
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| 103 | no leading whitespace
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| 104 | '''
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| 105 |
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| 106 | Again, you can omit the `r` prefix if there's no backslash, because it's not
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| 107 | ambiguous:
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| 108 |
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| 109 | var myRaw = '''
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| 110 | raw string
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| 111 | no leading whitespace
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| 112 | '''
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| 113 |
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| 114 | ### str-template
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| 115 |
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| 116 | String templates use the same syntax as double-quoted strings:
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| 117 |
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| 118 | var mytemplate = ^"name = $name, age = $age"
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| 119 |
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| 120 | Related topics:
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| 121 |
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| 122 | - [Str => replace](chap-type-method.html#replace)
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| 123 | - [ysh-string](chap-expr-lang.html#ysh-string)
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| 124 |
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| 125 | ### list-literal
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| 126 |
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| 127 | Lists have a Python-like syntax:
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| 128 |
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| 129 | var mylist = ['one', 'two', 3]
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| 130 |
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| 131 | And a shell-like syntax:
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| 132 |
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| 133 | var list2 = %| one two |
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| 134 |
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| 135 | The shell-like syntax accepts the same syntax that a command can:
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| 136 |
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| 137 | ls $mystr @ARGV *.py {foo,bar}@example.com
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| 138 |
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| 139 | # Rather than executing ls, evaluate and store words
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| 140 | var cmd = :| ls $mystr @ARGV *.py {foo,bar}@example.com |
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| 141 |
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| 142 | ### dict-literal
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| 143 |
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| 144 | {name: 'value'}
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| 145 |
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| 146 | ### range
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| 147 |
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| 148 | A range is a sequence of numbers that can be iterated over:
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| 149 |
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| 150 | for i in (0 .. 3) {
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| 151 | echo $i
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| 152 | }
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| 153 | => 0
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| 154 | => 1
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| 155 | => 2
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| 156 |
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| 157 | As with slices, the last number isn't included. Idiom to iterate from 1 to n:
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| 158 |
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| 159 | for i in (1 .. n+1) {
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| 160 | echo $i
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| 161 | }
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| 162 |
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| 163 | ### block-literal
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| 164 |
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| 165 | var myblock = ^(echo $PWD)
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| 166 |
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| 167 | ### expr-lit
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| 168 |
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| 169 | var myexpr = ^[1 + 2*3]
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| 170 |
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| 171 | ## Operators
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| 172 |
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| 173 | <h3 id="concat">concat <code>++</code></h3>
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| 174 |
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| 175 | The concatenation operator works on strings:
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| 176 |
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| 177 | var s = 'hello'
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| 178 | var t = s ++ ' world'
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| 179 | = t
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| 180 | (Str) "hello world"
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| 181 |
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| 182 | and lists:
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| 183 |
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| 184 | var L = ['one', 'two']
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| 185 | var M = L ++ ['three', '4']
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| 186 | = M
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| 187 | (List) ["one", "two", "three", "4"]
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| 188 |
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| 189 | String interpolation can be nicer than `++`:
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| 190 |
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| 191 | var t2 = "${s} world" # same as t
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| 192 |
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| 193 | Likewise, splicing lists can be nicer:
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| 194 |
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| 195 | var M2 = :| @L three 4 | # same as M
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| 196 |
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| 197 | ### ysh-compare
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| 198 |
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| 199 | a == b # Python-like equality, no type conversion
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| 200 | 3 ~== 3.0 # True, type conversion
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| 201 | 3 ~== '3' # True, type conversion
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| 202 | 3 ~== '3.0' # True, type conversion
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| 203 |
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| 204 | ### ysh-logical
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| 205 |
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| 206 | not and or
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| 207 |
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| 208 | Note that these are distinct from `! && ||`.
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| 209 |
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| 210 | ### ysh-arith
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| 211 |
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| 212 | + - * / // % **
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| 213 |
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| 214 | ### ysh-bitwise
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| 215 |
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| 216 | ~ & | ^
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| 217 |
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| 218 | ### ysh-ternary
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| 219 |
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| 220 | Like Python:
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| 221 |
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| 222 | display = 'yes' if len(s) else 'empty'
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| 223 |
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| 224 | ### ysh-index
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| 225 |
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| 226 | Like Python:
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| 227 |
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| 228 | myarray[3]
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| 229 | mystr[3]
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| 230 |
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| 231 | TODO: Does string indexing give you an integer back?
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| 232 |
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| 233 | ### ysh-slice
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| 234 |
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| 235 | Like Python:
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| 236 |
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| 237 | myarray[1 : -1]
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| 238 | mystr[1 : -1]
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| 239 |
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| 240 | ### func-call
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| 241 |
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| 242 | Like Python:
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| 243 |
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| 244 | f(x, y)
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| 245 |
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| 246 | ### thin-arrow
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| 247 |
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| 248 | The thin arrow is for mutating methods:
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| 249 |
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| 250 | var mylist = ['bar']
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| 251 | call mylist->pop()
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| 252 |
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| 253 | <!--
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| 254 | TODO
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| 255 | var mydict = {name: 'foo'}
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| 256 | call mydict->erase('name')
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| 257 | -->
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| 258 |
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| 259 | ### fat-arrow
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| 260 |
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| 261 | The fat arrow is for transforming methods:
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| 262 |
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| 263 | if (s => startsWith('prefix')) {
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| 264 | echo 'yes'
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| 265 | }
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| 266 |
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| 267 | If the method lookup on `s` fails, it looks for free functions. This means it
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| 268 | can be used for "chaining" transformations:
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| 269 |
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| 270 | var x = myFunc() => list() => join()
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| 271 |
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| 272 | ### match-ops
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| 273 |
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| 274 | YSH has four pattern matching operators: `~ !~ ~~ !~~`.
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| 275 |
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| 276 | Does string match an **eggex**?
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| 277 |
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| 278 | var filename = 'x42.py'
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| 279 | if (filename ~ / d+ /) {
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| 280 | echo 'number'
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| 281 | }
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| 282 |
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| 283 | Does a string match a POSIX regular expression (ERE syntax)?
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| 284 |
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| 285 | if (filename ~ '[[:digit:]]+') {
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| 286 | echo 'number'
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| 287 | }
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| 288 |
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| 289 | Negate the result with the `!~` operator:
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| 290 |
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| 291 | if (filename !~ /space/ ) {
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| 292 | echo 'no space'
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| 293 | }
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| 294 |
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| 295 | if (filename !~ '[[:space:]]' ) {
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| 296 | echo 'no space'
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| 297 | }
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| 298 |
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| 299 | Does a string match a **glob**?
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| 300 |
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| 301 | if (filename ~~ '*.py') {
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| 302 | echo 'Python'
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| 303 | }
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| 304 |
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| 305 | if (filename !~~ '*.py') {
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| 306 | echo 'not Python'
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| 307 | }
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| 308 |
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| 309 | Take care not to confuse glob patterns and regular expressions.
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| 310 |
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| 311 | - Related doc: [YSH Regex API](../ysh-regex-api.html)
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| 312 |
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| 313 | ## Eggex
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| 314 |
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| 315 | ### re-literal
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| 316 |
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| 317 | An eggex literal looks like this:
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| 318 |
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| 319 | / expression ; flags ; translation preference /
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| 320 |
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| 321 | The flags and translation preference are both optional.
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| 322 |
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| 323 | Examples:
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| 324 |
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| 325 | var pat = / d+ / # => [[:digit:]]+
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| 326 |
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| 327 | You can specify flags passed to libc `regcomp()`:
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| 328 |
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| 329 | var pat = / d+ ; reg_icase reg_newline /
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| 330 |
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| 331 | You can specify a translation preference after a second semi-colon:
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| 332 |
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| 333 | var pat = / d+ ; ; ERE /
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| 334 |
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| 335 | Right now the translation preference does nothing. It could be used to
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| 336 | translate eggex to PCRE or Python syntax.
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| 337 |
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| 338 | - Related doc: [Egg Expressions](../eggex.html)
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| 339 |
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| 340 | ### re-primitive
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| 341 |
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| 342 | There are two kinds of eggex primitives.
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| 343 |
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| 344 | "Zero-width assertions" match a position rather than a character:
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| 345 |
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| 346 | %start # translates to ^
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| 347 | %end # translates to $
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| 348 |
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| 349 | Literal characters appear within **single** quotes:
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| 350 |
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| 351 | 'oh *really*' # translates to regex-escaped string
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| 352 |
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| 353 | Double-quoted strings are **not** eggex primitives. Instead, you can use
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| 354 | splicing of strings:
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| 355 |
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| 356 | var dq = "hi $name"
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| 357 | var eggex = / @dq /
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| 358 |
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| 359 | ### class-literal
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| 360 |
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| 361 | An eggex character class literal specifies a set. It can have individual
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| 362 | characters and ranges:
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| 363 |
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| 364 | [ 'x' 'y' 'z' a-f A-F 0-9 ] # 3 chars, 3 ranges
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| 365 |
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| 366 | Omit quotes on ASCII characters:
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| 367 |
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| 368 | [ x y z ] # avoid typing 'x' 'y' 'z'
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| 369 |
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| 370 | Sets of characters can be written as trings
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| 371 |
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| 372 | [ 'xyz' ] # any of 3 chars, not a sequence of 3 chars
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| 373 |
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| 374 | Backslash escapes are respected:
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| 375 |
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| 376 | [ \\ \' \" \0 ]
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| 377 | [ \xFF \u0100 ]
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| 378 |
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| 379 | Splicing:
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| 380 |
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| 381 | [ @str_var ]
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| 382 |
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| 383 | Negation always uses `!`
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| 384 |
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| 385 | ![ a-f A-F 'xyz' @str_var ]
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| 386 |
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| 387 | ### named-class
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| 388 |
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| 389 | Perl-like shortcuts for sets of characters:
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| 390 |
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| 391 | [ dot ] # => .
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| 392 | [ digit ] # => [[:digit:]]
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| 393 | [ space ] # => [[:space:]]
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| 394 | [ word ] # => [[:alpha:]][[:digit:]]_
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| 395 |
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| 396 | Abbreviations:
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| 397 |
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| 398 | [ d s w ] # Same as [ digit space word ]
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| 399 |
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| 400 | Valid POSIX classes:
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| 401 |
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| 402 | alnum cntrl lower space
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| 403 | alpha digit print upper
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| 404 | blank graph punct xdigit
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| 405 |
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| 406 | Negated:
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| 407 |
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| 408 | !digit !space !word
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| 409 | !d !s !w
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| 410 | !alnum # etc.
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| 411 |
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| 412 | ### re-repeat
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| 413 |
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| 414 | Eggex repetition looks like POSIX syntax:
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| 415 |
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| 416 | / 'a'? / # zero or one
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| 417 | / 'a'* / # zero or more
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| 418 | / 'a'+ / # one or more
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| 419 |
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| 420 | Counted repetitions:
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| 421 |
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| 422 | / 'a'{3} / # exactly 3 repetitions
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| 423 | / 'a'{2,4} / # between 2 to 4 repetitions
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| 424 |
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| 425 | ### re-compound
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| 426 |
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| 427 | Sequence expressions with a space:
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| 428 |
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| 429 | / word digit digit / # Matches 3 characters in sequence
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| 430 | # Examples: a42, b51
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| 431 |
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| 432 | (Compare `/ [ word digit ] /`, which is a set matching 1 character.)
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| 433 |
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| 434 | Alternation with `|`:
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| 435 |
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| 436 | / word | digit / # Matches 'a' OR '9', for example
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| 437 |
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| 438 | Grouping with parentheses:
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| 439 |
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| 440 | / (word digit) | \\ / # Matches a9 or \
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| 441 |
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| 442 | ### re-capture
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| 443 |
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| 444 | To retrieve a substring of a string that matches an Eggex, use a "capture
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| 445 | group" like `<capture ...>`.
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| 446 |
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| 447 | Here's an eggex with a **positional** capture:
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| 448 |
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| 449 | var pat = / 'hi ' <capture d+> / # access with _group(1)
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| 450 | # or Match => _group(1)
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| 451 |
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| 452 | Captures can be **named**:
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| 453 |
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| 454 | <capture d+ as month> # access with _group('month')
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| 455 | # or Match => group('month')
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| 456 |
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| 457 | Captures can also have a type **conversion func**:
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| 458 |
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| 459 | <capture d+ : int> # _group(1) returns Int
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| 460 |
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| 461 | <capture d+ as month: int> # _group('month') returns Int
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| 462 |
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| 463 | Related docs and help topics:
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| 464 |
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| 465 | - [YSH Regex API](../ysh-regex-api.html)
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| 466 | - [`_group()`](chap-builtin-func.html#_group)
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| 467 | - [`Match => group()`](chap-type-method.html#group)
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| 468 |
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| 469 | ### re-splice
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| 470 |
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| 471 | To build an eggex out of smaller expressions, you can **splice** eggexes
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| 472 | together:
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| 473 |
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| 474 | var D = / [0-9][0-9] /
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| 475 | var time = / @D ':' @D / # [0-9][0-9]:[0-9][0-9]
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| 476 |
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| 477 | If the variable begins with a capital letter, you can omit `@`:
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| 478 |
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| 479 | var ip = / D ':' D /
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| 480 |
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| 481 | You can also splice a string:
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| 482 |
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| 483 | var greeting = 'hi'
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| 484 | var pat = / @greeting ' world' / # hi world
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| 485 |
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| 486 | Splicing is **not** string concatenation; it works on eggex subtrees.
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| 487 |
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| 488 | ### re-flags
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| 489 |
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| 490 | Valid ERE flags, which are passed to libc's `regcomp()`:
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| 491 |
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| 492 | - `reg_icase` aka `i` - ignore case
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| 493 | - `reg_newline` - 4 matching changes related to newlines
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| 494 |
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| 495 | See `man regcomp`.
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| 496 |
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| 497 | ### re-multiline
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| 498 |
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| 499 | Multi-line eggexes aren't yet implemented. Splicing makes it less necessary:
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| 500 |
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| 501 | var Name = / <capture [a-z]+ as name> /
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| 502 | var Num = / <capture d+ as num> /
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| 503 | var Space = / <capture s+ as space> /
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| 504 |
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| 505 | # For variables named like CapWords, splicing @Name doesn't require @
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| 506 | var lexer = / Name | Num | Space /
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