Starship context_alias uses Rust regex — $1t is parsed as group named "1t", not group 1 followed by "t"

2026-06-18 (3m ago)6 views

I was configuring Starship's [kubernetes] module to show short aliases for my CaaS cluster contexts. The context names look like:

region1-caas1-cluster1-prod-000123-myservice-api

I wanted to extract the region digit (1) and cluster digit (1) and produce r1c1. So I wrote:

{ context_pattern = "region(\\d+)-caas\\d+-cluster(\\d+)-.+-prod-\\d+-(\\w+)-(api|ui)",
  context_alias = "r$1c$2/$3[$4]" }

The result was j1/domestic[api] — the t and the second digit were silently dropped.

Turns out, Starship uses Rust's regex crate for replacement, and in Rust regex substitution syntax, $name consumes all alphanumeric characters (and underscores) as the group name. So $1t is interpreted as a reference to a capture group literally named "1t" — which doesn't exist — and expands to nothing.

The fix is to use ${n} to explicitly delimit the group reference:

context_alias = "r${1}c${2}/${3}[${4}]"

This is the same syntax Rust's Regex::replace uses when you need a literal character immediately after a backreference. The curly braces close the group name so everything after } is treated as literal text.

Other gotcha in the same session: no lookaround

Starship also doesn't support lookahead or lookbehind (Rust's regex crate deliberately excludes them for guaranteed linear-time matching). I had tried:

context_pattern = "...(?<!non)-prod-..."

to distinguish prod from non-prod contexts, and got:

[WARN] - (starship::modules::kubernetes): Could not compile regular expression: look-around, including look-ahead and look-behind, is not supported

The fix is simpler anyway: put the non-prod rule before the prod rule in the contexts list. Starship uses first-match, so once non-prod is consumed, the prod pattern never sees those contexts.