1 | ---
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2 | title: YSH Expression Language (Oils Reference)
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3 | all_docs_url: ..
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4 | body_css_class: width40
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5 | default_highlighter: oils-sh
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6 | preserve_anchor_case: yes
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7 | ---
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8 |
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9 | <div class="doc-ref-header">
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10 |
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11 | [Oils Reference](index.html) —
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12 | Chapter **YSH Expression Language**
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13 |
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14 | </div>
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15 |
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16 | This chapter describes the YSH expression language, which includes [Egg
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17 | Expressions]($xref:eggex).
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18 |
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19 | <div id="dense-toc">
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20 | </div>
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21 |
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22 | ## Assignment
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23 |
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24 | ### assign
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25 |
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26 | The `=` operator is used with assignment keywords:
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27 |
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28 | var x = 42
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29 | setvar x = 43
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30 |
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31 | const y = 'k'
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32 |
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33 | setglobal z = 'g'
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34 |
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35 | ### aug-assign
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36 |
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37 | The augmented assignment operators are:
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38 |
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39 | += -= *= /= **= //= %=
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40 | &= |= ^= <<= >>=
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41 |
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42 | They are used with `setvar` and `setglobal`. For example:
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43 |
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44 | setvar x += 2
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45 |
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46 | is the same as:
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47 |
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48 | setvar x = x + 2
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49 |
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50 | Likewise, these are the same:
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51 |
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52 | setglobal a[i] -= 1
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53 |
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54 | setglobal a[i] = a[i] - 1
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55 |
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56 | ## Literals
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57 |
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58 | ### atom-literal
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59 |
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60 | YSH uses JavaScript-like spellings for these three "atoms":
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61 |
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62 | null # type Null
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63 | true false # type Bool
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64 |
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65 | Note: to signify "no value", you may sometimes use an empty string `''`,
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66 | instead of `null`.
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67 |
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68 | ### int-literal
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69 |
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70 | Examples of integer literals:
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71 |
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72 | var decimal = 42
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73 | var big = 42_000
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74 |
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75 | var hex = 0x0010_ffff
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76 |
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77 | var octal = 0o755
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78 |
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79 | var binary = 0b0001_0000
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80 |
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81 | ### float-lit
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82 |
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83 | Examples of float literals:
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84 |
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85 | var myfloat = 3.14
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86 |
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87 | var f2 = -1.5e-100
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88 |
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89 | ### char-literal
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90 |
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91 | Three kinds of unquoted backslash escapes are allowed in expression mode. They
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92 | match what's available in quoted J8-style strings:
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93 |
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94 | var backslash = \\
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95 | var quotes = \' ++ \" # same as u'\'' ++ '"'
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96 |
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97 | var mu = \u{3bc} # same as u'\u{3bc}'
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98 |
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99 | var nul = \y00 # same as b'\y00'
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100 |
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101 | ### ysh-string
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102 |
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103 | YSH has single and double-quoted strings borrowed from Bourne shell, and
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104 | C-style strings borrowed from J8 Notation.
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105 |
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106 | Double quoted strings respect `$` interpolation:
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107 |
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108 | var dq = "hello $world and $(hostname)"
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109 |
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110 | You can add a `$` before the left quote to be explicit: `$"x is $x"` rather
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111 | than `"x is $x"`.
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112 |
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113 | Single quoted strings may be raw:
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114 |
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115 | var s = r'line\n' # raw string means \n is literal, NOT a newline
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116 |
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117 | Or *J8 strings* with backslash escapes:
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118 |
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119 | var s = u'line\n \u{3bc}' # unicode string means \n is a newline
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120 | var s = b'line\n \u{3bc} \yff' # same thing, but also allows bytes
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121 |
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122 | Both `u''` and `b''` strings evaluate to the single `Str` type. The difference
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123 | is that `b''` strings allow the `\yff` byte escape.
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124 |
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125 | #### Notes
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126 |
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127 | There's no way to express a single quote in raw strings. Use one of the other
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128 | forms instead:
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129 |
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130 | var sq = "single quote: ' "
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131 | var sq = u'single quote: \' '
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132 |
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133 | Sometimes you can omit the `r`, e.g. where there are no backslashes and thus no
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134 | ambiguity:
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135 |
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136 | echo 'foo'
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137 | echo r'foo' # same thing
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138 |
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139 | The `u''` and `b''` strings are called *J8 strings* because the syntax in YSH
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140 | **code** matches JSON-like **data**.
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141 |
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142 | var strU = u'mu = \u{3bc}' # J8 string with escapes
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143 | var strB = b'bytes \yff' # J8 string that can express byte strings
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144 |
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145 | More examples:
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146 |
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147 | var myRaw = r'[a-z]\n' # raw strings can be used for regexes (not
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148 | # eggexes)
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149 |
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150 | ### triple-quoted
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151 |
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152 | Triple-quoted string literals have leading whitespace stripped on each line.
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153 | They come in the same variants:
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154 |
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155 | var dq = """
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156 | hello $world and $(hostname)
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157 | no leading whitespace
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158 | """
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159 |
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160 | var myRaw = r'''
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161 | raw string
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162 | no leading whitespace
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163 | '''
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164 |
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165 | var strU = u'''
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166 | string that happens to be unicode \u{3bc}
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167 | no leading whitespace
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168 | '''
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169 |
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170 | var strB = b'''
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171 | string that happens to be bytes \u{3bc} \yff
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172 | no leading whitespace
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173 | '''
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174 |
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175 | Again, you can omit the `r` prefix if there's no backslash, because it's not
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176 | ambiguous:
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177 |
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178 | var myRaw = '''
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179 | raw string
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180 | no leading whitespace
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181 | '''
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182 |
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183 | ### str-template
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184 |
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185 | String templates use the same syntax as double-quoted strings:
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186 |
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187 | var mytemplate = ^"name = $name, age = $age"
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188 |
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189 | Related topics:
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190 |
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191 | - [Str => replace](chap-type-method.html#replace)
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192 | - [ysh-string](chap-expr-lang.html#ysh-string)
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193 |
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194 | ### list-literal
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195 |
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196 | Lists have a Python-like syntax:
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197 |
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198 | var mylist = ['one', 'two', [42, 43]]
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199 |
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200 | And a shell-like syntax:
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201 |
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202 | var list2 = :| one two |
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203 |
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204 | The shell-like syntax accepts the same syntax as a simple command:
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205 |
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206 | ls $mystr @ARGV *.py {foo,bar}@example.com
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207 |
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208 | # Rather than executing ls, evaluate words into a List
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209 | var cmd = :| ls $mystr @ARGV *.py {foo,bar}@example.com |
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210 |
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211 | ### dict-literal
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212 |
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213 | Dicts look like JavaScript.
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214 |
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215 | var d = {
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216 | key1: 'value', # key can be unquoted if it looks like a var name
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217 | 'key2': 42, # or quote it
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218 |
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219 | ['key2' ++ suffix]: 43, # bracketed expression
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220 | }
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221 |
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222 | Omitting a value means that the corresponding key takes the value of a var of
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223 | the same name:
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224 |
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225 | ysh$ var x = 42
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226 | ysh$ var y = 43
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227 |
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228 | ysh$ var d = {x, y} # values omitted
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229 | ysh$ = d
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230 | (Dict) {x: 42, y: 43}
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231 |
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232 | ### range
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233 |
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234 | A range is a sequence of numbers that can be iterated over:
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235 |
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236 | for i in (0 .. 3) {
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237 | echo $i
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238 | }
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239 | => 0
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240 | => 1
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241 | => 2
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242 |
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243 | As with slices, the last number isn't included. To iterate from 1 to n, you
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244 | can use this idiom:
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245 |
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246 | for i in (1 .. n+1) {
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247 | echo $i
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248 | }
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249 |
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250 | ### block-expr
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251 |
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252 | In YSH expressions, we use `^()` to create a [Command][] object:
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253 |
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254 | var myblock = ^(echo $PWD; ls *.txt)
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255 |
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256 | It's more common for [Command][] objects to be created with block arguments,
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257 | which are not expressions:
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258 |
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259 | cd /tmp {
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260 | echo $PWD
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261 | ls *.txt
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262 | }
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263 |
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264 | [Command]: chap-type-method.html#Command
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265 |
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266 | ### expr-literal
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267 |
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268 | An expression literal is an object that holds an unevaluated expression:
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269 |
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270 | var myexpr = ^[1 + 2*3]
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271 |
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272 | [Expr]: chap-type-method.html#Expr
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273 |
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274 | ## Operators
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275 |
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276 | ### op-precedence
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277 |
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278 | YSH operator precedence is identical to Python's operator precedence.
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279 |
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280 | New operators:
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281 |
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282 | - `++` has the same precedence as `+`
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283 | - `->` and `=>` have the same precedence as `.`
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284 |
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285 | <!-- TODO: show grammar -->
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286 |
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287 |
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288 | <h3 id="concat">concat <code>++</code></h3>
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289 |
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290 | The concatenation operator works on `Str` objects:
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291 |
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292 | ysh$ var s = 'hello'
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293 | ysh$ var t = s ++ ' world'
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294 |
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295 | ysh$ = t
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296 | (Str) "hello world"
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297 |
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298 | and `List` objects:
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299 |
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300 | ysh$ var L = ['one', 'two']
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301 | ysh$ var M = L ++ ['three', '4']
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302 |
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303 | ysh$ = M
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304 | (List) ["one", "two", "three", "4"]
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305 |
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306 | String interpolation can be nicer than `++`:
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307 |
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308 | var t2 = "${s} world" # same as t
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309 |
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310 | Likewise, splicing lists can be nicer:
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311 |
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312 | var M2 = :| @L three 4 | # same as M
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313 |
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314 | ### ysh-equals
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315 |
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316 | YSH has strict equality:
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317 |
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318 | a === b # Python-like, without type conversion
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319 | a !== b # negated
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320 |
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321 | And type converting equality:
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322 |
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323 | '3' ~== 3 # True, type conversion
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324 |
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325 | The `~==` operator expects a string as the left operand.
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326 |
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327 | ---
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328 |
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329 | Note that:
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330 |
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331 | - `3 === 3.0` is false because integers and floats are different types, and
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332 | there is no type conversion.
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333 | - `3 ~== 3.0` is an error, because the left operand isn't a string.
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334 |
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335 | You may want to use explicit `int()` and `float()` to convert numbers, and then
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336 | compare them.
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337 |
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338 | ---
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339 |
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340 | Compare objects for identity with `is`:
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341 |
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342 | ysh$ var d = {}
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343 | ysh$ var e = d
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344 |
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345 | ysh$ = d is d
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346 | (Bool) true
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347 |
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348 | ysh$ = d is {other: 'dict'}
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349 | (Bool) false
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350 |
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351 | To negate `is`, use `is not` (like Python:
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352 |
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353 | ysh$ d is not {other: 'dict'}
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354 | (Bool) true
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355 |
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356 | ### ysh-in
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357 |
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358 | The `in` operator tests if a key is in a dictionary:
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359 |
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360 | var d = {k: 42}
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361 | if ('k' in d) {
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362 | echo yes
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363 | } # => yes
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364 |
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365 | Unlike Python, `in` doesn't work on `Str` and `List` instances. This because
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366 | those operations take linear time rather than constant time (O(n) rather than
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367 | O(1)).
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368 |
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369 | TODO: Use `includes() / contains()` methods instead.
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370 |
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371 | ### ysh-compare
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372 |
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373 | The comparison operators apply to integers or floats:
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374 |
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375 | 4 < 4 # => false
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376 | 4 <= 4 # => true
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377 |
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378 | 5.0 > 5.0 # => false
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379 | 5.0 >= 5.0 # => true
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380 |
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381 | Example in context:
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382 |
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383 | if (x < 0) {
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384 | echo 'x is negative'
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385 | }
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386 |
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387 | ### ysh-logical
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388 |
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389 | The logical operators take boolean operands, and are spelled like Python:
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390 |
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391 | not
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392 | and or
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393 |
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394 | Note that they are distinct from `! && ||`, which are part of the [command
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395 | language](chap-cmd-lang.html).
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396 |
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397 | ### ysh-arith
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398 |
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399 | YSH supports most of the arithmetic operators from Python. Notably, `/` and `%`
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400 | differ from Python as [they round toward zero, not negative
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401 | infinity](https://www.oilshell.org/blog/2024/03/release-0.21.0.html#integers-dont-do-whatever-python-or-c-does).
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402 |
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403 | Use `+ - *` for `Int` or `Float` addition, subtraction and multiplication. If
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404 | any of the operands are `Float`s, then the output will also be a `Float`.
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405 |
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406 | Use `/` and `//` for `Float` division and `Int` division, respectively. `/`
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407 | will _always_ result in a `Float`, meanwhile `//` will _always_ result in an
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408 | `Int`.
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409 |
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410 | = 1 / 2 # => (Float) 0.5
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411 | = 1 // 2 # => (Int) 0
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412 |
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413 | Use `%` to compute the _remainder_ of integer division. The left operand must
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414 | be an `Int` and the right a _positive_ `Int`.
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415 |
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416 | = 1 % 2 # -> (Int) 1
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417 | = -4 % 2 # -> (Int) 0
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418 |
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419 | Use `**` for exponentiation. The left operand must be an `Int` and the right a
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420 | _positive_ `Int`.
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421 |
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422 | All arithmetic operators may coerce either of their operands from strings to a
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423 | number, provided those strings are formatted as numbers.
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424 |
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425 | = 10 + '1' # => (Int) 11
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426 |
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427 | Operators like `+ - * /` will coerce strings to _either_ an `Int` or `Float`.
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428 | However, operators like `// ** %` and bit shifts will coerce strings _only_ to
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429 | an `Int`.
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430 |
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431 | = '1.14' + '2' # => (Float) 3.14
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432 | = '1.14' % '2' # Type Error: Left operand is a Str
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433 |
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434 | ### ysh-bitwise
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435 |
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436 | Bitwise operators are like Python and C:
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437 |
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438 | ~ # unary complement
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439 |
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440 | & | ^ # binary and, or, xor
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441 |
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442 | >> << # bit shift
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443 |
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444 | ### ysh-ternary
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445 |
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446 | The ternary operator is borrowed from Python:
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447 |
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448 | display = 'yes' if len(s) else 'empty'
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449 |
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450 | ### ysh-index
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451 |
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452 | `Str` objects can be indexed by byte:
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453 |
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454 | ysh$ var s = 'cat'
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455 | ysh$ = mystr[1]
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456 | (Str) 'a'
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457 |
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458 | ysh$ = mystr[-1] # index from the end
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459 | (Str) 't'
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460 |
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461 | `List` objects:
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462 |
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463 | ysh$ var mylist = [1, 2, 3]
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464 | ysh$ = mylist[2]
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465 | (Int) 3
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466 |
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467 | `Dict` objects are indexed by string key:
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468 |
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469 | ysh$ var mydict = {'key': 42}
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470 | ysh$ = mydict['key']
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471 | (Int) 42
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472 |
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473 | ### ysh-attr
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474 |
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475 | The expression `mydict.key` is short for `mydict['key']`.
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476 |
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477 | (Like JavaScript, but unlike Python.)
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478 |
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479 | ### ysh-slice
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480 |
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481 | Slicing gives you a subsequence of a `Str` or `List`, as in Python.
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482 |
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483 | Negative indices are relative to the end.
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484 |
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485 | String example:
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486 |
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487 | $ var s = 'spam eggs'
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488 | $ pp line (s[1:-1])
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489 | (Str) "pam egg"
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490 |
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491 | $ echo "x $[s[2:]]"
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492 | x am eggs
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493 |
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494 | List example:
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495 |
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496 | $ var foods = ['ale', 'bean', 'corn']
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497 | $ pp line (foods[-2:])
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498 | (List) ["bean","corn"]
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499 |
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500 | $ write -- @[foods[:2]]
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501 | ale
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502 | bean
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503 |
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504 | ### func-call
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505 |
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506 | A function call expression looks like Python:
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507 |
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508 | ysh$ = f('s', 't', named=42)
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509 |
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510 | A semicolon `;` can be used after positional args and before named args, but
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511 | isn't always required:
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512 |
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513 | ysh$ = f('s', 't'; named=42)
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514 |
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515 | In these cases, the `;` is necessary:
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516 |
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517 | ysh$ = f(...args; ...kwargs)
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518 |
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519 | ysh$ = f(42, 43; ...kwargs)
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520 |
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521 | ### thin-arrow
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522 |
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523 | The thin arrow is for mutating methods:
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524 |
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525 | var mylist = ['bar']
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526 | call mylist->pop()
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527 |
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528 | <!--
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529 | TODO
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530 | var mydict = {name: 'foo'}
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531 | call mydict->erase('name')
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532 | -->
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533 |
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534 | ### fat-arrow
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535 |
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536 | The fat arrow is for transforming methods:
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537 |
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538 | if (s => startsWith('prefix')) {
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539 | echo 'yes'
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540 | }
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541 |
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542 | If the method lookup on `s` fails, it looks for free functions. This means it
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543 | can be used for "chaining" transformations:
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544 |
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545 | var x = myFunc() => list() => join()
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546 |
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547 | ### match-ops
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548 |
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549 | YSH has four pattern matching operators: `~ !~ ~~ !~~`.
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550 |
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551 | Does string match an **eggex**?
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552 |
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553 | var filename = 'x42.py'
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554 | if (filename ~ / d+ /) {
|
555 | echo 'number'
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556 | }
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557 |
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558 | Does a string match a POSIX regular expression (ERE syntax)?
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559 |
|
560 | if (filename ~ '[[:digit:]]+') {
|
561 | echo 'number'
|
562 | }
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563 |
|
564 | Negate the result with the `!~` operator:
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565 |
|
566 | if (filename !~ /space/ ) {
|
567 | echo 'no space'
|
568 | }
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569 |
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570 | if (filename !~ '[[:space:]]' ) {
|
571 | echo 'no space'
|
572 | }
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573 |
|
574 | Does a string match a **glob**?
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575 |
|
576 | if (filename ~~ '*.py') {
|
577 | echo 'Python'
|
578 | }
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579 |
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580 | if (filename !~~ '*.py') {
|
581 | echo 'not Python'
|
582 | }
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583 |
|
584 | Take care not to confuse glob patterns and regular expressions.
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585 |
|
586 | - Related doc: [YSH Regex API](../ysh-regex-api.html)
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587 |
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588 | ## Eggex
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589 |
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590 | ### re-literal
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591 |
|
592 | An eggex literal looks like this:
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593 |
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594 | / expression ; flags ; translation preference /
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595 |
|
596 | The flags and translation preference are both optional.
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597 |
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598 | Examples:
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599 |
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600 | var pat = / d+ / # => [[:digit:]]+
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601 |
|
602 | You can specify flags passed to libc `regcomp()`:
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603 |
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604 | var pat = / d+ ; reg_icase reg_newline /
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605 |
|
606 | You can specify a translation preference after a second semi-colon:
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607 |
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608 | var pat = / d+ ; ; ERE /
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609 |
|
610 | Right now the translation preference does nothing. It could be used to
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611 | translate eggex to PCRE or Python syntax.
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612 |
|
613 | - Related doc: [Egg Expressions](../eggex.html)
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614 |
|
615 | ### re-primitive
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616 |
|
617 | There are two kinds of eggex primitives.
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618 |
|
619 | "Zero-width assertions" match a position rather than a character:
|
620 |
|
621 | %start # translates to ^
|
622 | %end # translates to $
|
623 |
|
624 | Literal characters appear within **single** quotes:
|
625 |
|
626 | 'oh *really*' # translates to regex-escaped string
|
627 |
|
628 | Double-quoted strings are **not** eggex primitives. Instead, you can use
|
629 | splicing of strings:
|
630 |
|
631 | var dq = "hi $name"
|
632 | var eggex = / @dq /
|
633 |
|
634 | ### class-literal
|
635 |
|
636 | An eggex character class literal specifies a set. It can have individual
|
637 | characters and ranges:
|
638 |
|
639 | [ 'x' 'y' 'z' a-f A-F 0-9 ] # 3 chars, 3 ranges
|
640 |
|
641 | Omit quotes on ASCII characters:
|
642 |
|
643 | [ x y z ] # avoid typing 'x' 'y' 'z'
|
644 |
|
645 | Sets of characters can be written as strings
|
646 |
|
647 | [ 'xyz' ] # any of 3 chars, not a sequence of 3 chars
|
648 |
|
649 | Backslash escapes are respected:
|
650 |
|
651 | [ \\ \' \" \0 ]
|
652 | [ \xFF \u{3bc} ]
|
653 |
|
654 | (Note that we don't use `\yFF`, as in J8 strings.)
|
655 |
|
656 | Splicing:
|
657 |
|
658 | [ @str_var ]
|
659 |
|
660 | Negation always uses `!`
|
661 |
|
662 | ![ a-f A-F 'xyz' @str_var ]
|
663 |
|
664 | ### named-class
|
665 |
|
666 | Perl-like shortcuts for sets of characters:
|
667 |
|
668 | [ dot ] # => .
|
669 | [ digit ] # => [[:digit:]]
|
670 | [ space ] # => [[:space:]]
|
671 | [ word ] # => [[:alpha:]][[:digit:]]_
|
672 |
|
673 | Abbreviations:
|
674 |
|
675 | [ d s w ] # Same as [ digit space word ]
|
676 |
|
677 | Valid POSIX classes:
|
678 |
|
679 | alnum cntrl lower space
|
680 | alpha digit print upper
|
681 | blank graph punct xdigit
|
682 |
|
683 | Negated:
|
684 |
|
685 | !digit !space !word
|
686 | !d !s !w
|
687 | !alnum # etc.
|
688 |
|
689 | ### re-repeat
|
690 |
|
691 | Eggex repetition looks like POSIX syntax:
|
692 |
|
693 | / 'a'? / # zero or one
|
694 | / 'a'* / # zero or more
|
695 | / 'a'+ / # one or more
|
696 |
|
697 | Counted repetitions:
|
698 |
|
699 | / 'a'{3} / # exactly 3 repetitions
|
700 | / 'a'{2,4} / # between 2 to 4 repetitions
|
701 |
|
702 | ### re-compound
|
703 |
|
704 | Sequence expressions with a space:
|
705 |
|
706 | / word digit digit / # Matches 3 characters in sequence
|
707 | # Examples: a42, b51
|
708 |
|
709 | (Compare `/ [ word digit ] /`, which is a set matching 1 character.)
|
710 |
|
711 | Alternation with `|`:
|
712 |
|
713 | / word | digit / # Matches 'a' OR '9', for example
|
714 |
|
715 | Grouping with parentheses:
|
716 |
|
717 | / (word digit) | \\ / # Matches a9 or \
|
718 |
|
719 | ### re-capture
|
720 |
|
721 | To retrieve a substring of a string that matches an Eggex, use a "capture
|
722 | group" like `<capture ...>`.
|
723 |
|
724 | Here's an eggex with a **positional** capture:
|
725 |
|
726 | var pat = / 'hi ' <capture d+> / # access with _group(1)
|
727 | # or Match => _group(1)
|
728 |
|
729 | Captures can be **named**:
|
730 |
|
731 | <capture d+ as month> # access with _group('month')
|
732 | # or Match => group('month')
|
733 |
|
734 | Captures can also have a type **conversion func**:
|
735 |
|
736 | <capture d+ : int> # _group(1) returns Int
|
737 |
|
738 | <capture d+ as month: int> # _group('month') returns Int
|
739 |
|
740 | Related docs and help topics:
|
741 |
|
742 | - [YSH Regex API](../ysh-regex-api.html)
|
743 | - [`_group()`](chap-builtin-func.html#_group)
|
744 | - [`Match => group()`](chap-type-method.html#group)
|
745 |
|
746 | ### re-splice
|
747 |
|
748 | To build an eggex out of smaller expressions, you can **splice** eggexes
|
749 | together:
|
750 |
|
751 | var D = / [0-9][0-9] /
|
752 | var time = / @D ':' @D / # [0-9][0-9]:[0-9][0-9]
|
753 |
|
754 | If the variable begins with a capital letter, you can omit `@`:
|
755 |
|
756 | var ip = / D ':' D /
|
757 |
|
758 | You can also splice a string:
|
759 |
|
760 | var greeting = 'hi'
|
761 | var pat = / @greeting ' world' / # hi world
|
762 |
|
763 | Splicing is **not** string concatenation; it works on eggex subtrees.
|
764 |
|
765 | ### re-flags
|
766 |
|
767 | Valid ERE flags, which are passed to libc's `regcomp()`:
|
768 |
|
769 | - `reg_icase` aka `i` - ignore case
|
770 | - `reg_newline` - 4 matching changes related to newlines
|
771 |
|
772 | See `man regcomp`.
|
773 |
|
774 | ### re-multiline
|
775 |
|
776 | Multi-line eggexes aren't yet implemented. Splicing makes it less necessary:
|
777 |
|
778 | var Name = / <capture [a-z]+ as name> /
|
779 | var Num = / <capture d+ as num> /
|
780 | var Space = / <capture s+ as space> /
|
781 |
|
782 | # For variables named like CapWords, splicing @Name doesn't require @
|
783 | var lexer = / Name | Num | Space /
|